feat(llvm/phi): Phase 277 P1 - fail-fast validation for PHI strict mode

## Summary
Implemented fail-fast validation for PHI ordering and value resolution in strict mode.

## Changes

### P1-1: Strict mode for "PHI after terminator"
- File: `src/llvm_py/phi_wiring/wiring.py::ensure_phi`
- Behavior: `NYASH_LLVM_PHI_STRICT=1` → RuntimeError if PHI created after terminator
- Default: Warning only (no regression)

### P1-2: Strict mode for "fallback 0"
- File: `src/llvm_py/phi_wiring/wiring.py::wire_incomings`
- Behavior: Strict mode forbids silent fallback to 0 (2 locations)
  - Location 1: Unresolvable incoming value
  - Location 2: Type coercion failure
- Error messages point to next debug file: `llvm_builder.py::_value_at_end_i64`

### P1-3: Connect verify_phi_ordering() to execution path
- File: `src/llvm_py/builders/function_lower.py`
- Behavior: Verify PHI ordering after all instructions emitted
- Debug mode: Shows "✅ All N blocks have correct PHI ordering"
- Strict mode: Raises RuntimeError with block list if violations found

## Testing
✅ Test 1: strict=OFF - passes without errors
✅ Test 2: strict=ON - passes without errors (no violations in test fixtures)
✅ Test 3: debug mode - verify_phi_ordering() connected and running

## Scope
- LLVM harness (Python) changes only
- No new environment variables (uses existing 3 from Phase 277 P2)
- No JoinIR/Rust changes (root fix is Phase 279)
- Default behavior unchanged (strict mode opt-in)

## Next Steps
- Phase 278: Remove deprecated env var support
- Phase 279: Root fix - unify "2本のコンパイラ" pipelines

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/reference/language/LANGUAGE_REFERENCE_2025.md

@@ -72,7 +72,7 @@ Rust製インタープリターによる高性能実行と、直感的な構文
 ```nyash
 box ClassName {
     # フィールド宣言（Phase 12.7形式）
-    field1: TypeBox              # フィールド型アノテーション（P0では無視）
+    field1: TypeBox              # 型アノテーション（現状は契約として未強制。SSOT: docs/reference/language/types.md）
     field2: TypeBox
     field3                       # 型なしも可
     
@@ -133,10 +133,9 @@ static box Main {
 ```
 
 注意（静的Boxのメソッド引数規約）
-- 静的Boxのメソッドは先頭に暗黙の `self`（= Singleton）が存在する。
-- つまり呼び出し側は `Main.main()` のように書いても、意味論上は `Main.main(self, ...)` の形になる。
-- VM/MIR/LLVM いずれのバックエンドでも、この規約に基づき引数個数（arity）を判定する。
-- 開発時のガイドライン: 静的Box内に定義する全メソッドは「先頭に `self` を取る」形で設計すること（将来の最適化や検証で一貫性を保つため）。
+- 静的Boxのメソッドは **暗黙の receiver（`me/self`）を持たない**（既定）。
+- 呼び出し側の `Main.main()` は、`Main.main/<arity>` のような **receiver無しの関数呼び出し**へ正規化される。
+- 静的Box内で `me/this` を instance receiver として扱うのは禁止（Fail-Fast）。必要なら `Main.some_static()` の形で呼び出す。
 
 📌 **構文に関する注意（`static method` について）**
 

docs/reference/language/README.md

@@ -19,6 +19,9 @@ Imports and namespaces
 Variables and scope
 - See: reference/language/variables-and-scope.md — Block-scoped locals, assignment resolution, and strong/weak reference guidance.
 
+Type system (SSOT)
+- See: reference/language/types.md — runtime truthiness, `+`/compare/equality semantics, and the role/limits of MIR type facts.
+
 Grammar (EBNF)
 - See: reference/language/EBNF.md — Stage‑2 grammar specification used by parser implementations.
  - Unified Members (stored/computed/once/birth_once): see reference/language/EBNF.md “Box Members (Phase 15)” and the Language Reference section. Default ON (disable with `NYASH_ENABLE_UNIFIED_MEMBERS=0`).

docs/reference/language/quick-reference.md

@@ -40,21 +40,18 @@ Semicolons and ASI (Automatic Semicolon Insertion)
   - Ambiguous continuations; parser must Fail‑Fast with a clear message.
 
 Truthiness (boolean context)
-- `Bool` → itself
-- `Integer` → `0` is false; non‑zero is true
-- `String` → empty string is false; otherwise true
-- `Array`/`Map` → non‑null is true (size is not consulted)
-- `null`/`void` → false
+- SSOT: `reference/language/types.md`（runtime truthiness）
+- 実行上は `Bool/Integer/Float/String/Void` が中心。`BoxRef` は一部のコアBoxのみ許可され、その他は `TypeError`（Fail-Fast）。
 
 Equality and Comparison
-- `==` and `!=` compare primitive values (Integer/Bool/String). No implicit cross‑type coercion.
-- Box/Instance comparisons should use explicit methods (`equals`), or be normalized by the builder.
-- Compare operators `< <= > >=` are defined on integers (MVP).
+- SSOT: `reference/language/types.md`（`==`/`!=` と `< <= > >=` の runtime 仕様）
+- `==` は一部の cross-kind（`Integer↔Bool`, `Integer↔Float`）を best-effort で扱う。その他は `false`。
+- `< <= > >=` は `Integer/Float/String` の **同型同士**のみ（異型は `TypeError`）。
 
 String and Numeric `+`
-- If either side is `String`, `+` is string concatenation.
-- If both sides are numeric, `+` is addition.
-- Other mixes are errors (dev: warn; prod: error) — keep it explicit（必要なら `str(x)` を使う）。
+- SSOT: `reference/language/types.md`（runtime `+` 仕様）
+- `Integer+Integer`, `Float+Float` は加算。片側が `String` なら文字列連結（相手は文字列化）。
+- それ以外（例: `Integer+Bool`, `Integer+Float`）は `TypeError`（Fail-Fast）。
 
 Blocks and Control
 - `if (cond) { ... } [else { ... }]`

docs/reference/language/types.md

@@ -0,0 +1,133 @@
+# Type System (SSOT)
+
+Status: Draft / active (2025-12)
+
+This document defines the **current, executable** type semantics of Nyash/Hakorune.
+Implementation is currently anchored to the Rust VM (`src/backend/mir_interpreter/*`).
+
+If another backend differs from this document, treat it as a bug unless explicitly noted.
+
+Coercion SSOT status:
+- Decisions: `docs/development/current/main/phases/phase-274/P3-DECISIONS.md`
+- Implemented (VM + LLVM parity): Phase 275 P0
+
+---
+
+## 1. Big Picture
+
+- Nyash is **dynamically typed**: runtime values carry a tag (Integer/Float/Bool/String/Void/BoxRef/Future).
+- `local` declares a variable; **re-assignment is allowed** (single keyword policy).
+- There is currently no static type checker. Some parts of MIR carry **type facts** as metadata for optimization / routing, not for semantics.
+
+Terminology (SSOT):
+- **Runtime type**: what the VM executes on (`VMValue`).
+- **MIR type facts**: builder annotations (`MirType`, `value_types`, `value_origin_newbox`, `TypeCertainty`).
+
+---
+
+## 2. Variables and Re-assignment
+
+- `local x` / `local x = expr` introduces a mutable local variable.
+- Re-assignment is always allowed: `x = expr`.
+- “Immutable locals” (let/const) are not part of the language today; they can be introduced later as lint/strict checks without changing core semantics.
+
+Note: Field type annotations like `field: TypeBox` exist in syntax, but are currently **not enforced** as a type contract (docs-only) — see `LANGUAGE_REFERENCE_2025.md`.
+
+---
+
+## 3. Boolean Context (truthiness)
+
+Boolean context means:
+- `if (cond) { ... }`
+- `loop(cond) { ... }`
+- `!cond`
+- branch conditions generated from `&&` / `||` lowering
+
+Runtime rule (SSOT) is implemented by `to_bool_vm` (`src/backend/abi_util.rs`):
+
+- `Bool` → itself
+- `Integer` → `0` is false; non-zero is true
+- `Float` → `0.0` is false; non-zero is true
+- `String` → empty string is false; otherwise true
+- `Void` → **TypeError** (fail-fast)
+- `BoxRef`:
+  - bridge boxes only:
+    - `BoolBox` / `IntegerBox` / `StringBox` are unboxed and coerced like their primitive equivalents
+    - `VoidBox` is treated as `Void` → **TypeError**
+  - other BoxRef types → **TypeError**
+- `Future` → error (`TypeError`)
+
+This is intentionally fail-fast: “any object is truthy” is **not** assumed by default today.
+
+---
+
+## 4. Operators: `+`, comparisons, equality
+
+### 4.1 `+` (BinaryOp::Add)
+
+Runtime semantics are defined in the Rust VM (`eval_binop` in `src/backend/mir_interpreter/helpers.rs`):
+
+- Numeric addition:
+  - `Integer + Integer` → `Integer`
+  - `Float + Float` → `Float`
+- Numeric promotion:
+  - `Integer + Float` / `Float + Integer` → `Float` (promote int→float)
+- String concatenation:
+  - `String + String` → `String`
+- Other combinations are `TypeError` (e.g., `Integer + Bool`, `Integer + Float`, `Bool + Bool`, `BoxRef + ...`).
+
+Dev-only note:
+- `NYASH_VM_TOLERATE_VOID=1` (or `--dev` paths) may tolerate `Void` in some arithmetic as a safety valve; do not rely on it for spec.
+
+### 4.2 `< <= > >=` (CompareOp)
+
+Runtime semantics (`eval_cmp` in `src/backend/mir_interpreter/helpers.rs`):
+
+- `Integer <=> Integer`
+- `Float <=> Float`
+- `String <=> String` (lexicographic)
+- Other combinations are `TypeError`.
+
+### 4.3 `==` / `!=`
+
+Equality is implemented as `eq_vm` (`src/backend/abi_util.rs`) and used by comparisons:
+
+- Same-kind equality for primitives: `Integer/Float/Bool/String/Void`.
+- Cross-kind coercions (Number-only):
+  - `Integer` ↔ `Float` only, with a precise rule (avoid accidental true via float rounding)
+- `BoxRef == BoxRef` is pointer identity (`Arc::ptr_eq`).
+- `Void` is treated as equal to `BoxRef(VoidBox)` and `BoxRef(MissingBox)` for backward compatibility.
+- Other mixed kinds are `false` (not an error).
+
+Precise rule for `Int == Float` (or `Float == Int`):
+- if Float is NaN → false
+- if Float is finite, integral, and exactly representable as i64 → compare as i64
+- otherwise → false
+
+---
+
+## 5. `is` / `as` and TypeOp
+
+Source patterns like `x.is("TypeName")` / `x.as("TypeName")` are lowered to MIR `TypeOp(Check/Cast)` (see `src/mir/builder/exprs.rs`).
+
+Runtime behavior (Rust VM):
+- `TypeOp(Check, value, ty)` produces a `Bool`.
+- `TypeOp(Cast, value, ty)` returns the input value if it matches; otherwise `TypeError`.
+
+Backend note:
+- LLVM (llvmlite harness) must match this SSOT; if it differs, treat it as a bug.
+- Tracking: Phase 274 P2 (`docs/development/current/main/phases/phase-274/P2-INSTRUCTIONS.md`).
+
+---
+
+## 6. MIR Type Facts (non-semantic metadata)
+
+MIR has a lightweight type vocabulary (`MirType` in `src/mir/types.rs`) and per-value metadata:
+- `value_types: ValueId -> MirType` (type annotations / inferred hints)
+- `value_origin_newbox: ValueId -> BoxName` (origin facts for “Known receiver”)
+- `TypeCertainty::{Known, Union}` used by call routing (`src/mir/definitions/call_unified.rs`)
+
+Important rule:
+- These facts are for **optimization/routing** (e.g., Known-only rewrite, callee resolution) and must not be treated as semantic truth.
+
+If you need semantics, define it at the runtime layer (VM) and then optionally optimize by using these facts.