feat(mir): Phase 279 P0 - Type propagation pipeline SSOT unification

Eliminate "2本のコンパイラ" problem by unifying type propagation into single SSOT entry.

SSOT implementation:
- src/mir/type_propagation/pipeline.rs - SSOT type propagation pipeline
- TypePropagationPipeline::run() - Single entry point for all routes

Pipeline steps (fixed order):
1. Copy propagation (initial)
2. BinOp re-propagation (numeric promotion: Int+Float→Float)
3. Copy propagation (propagate promoted types)
4. PHI type inference (private step - cannot bypass)

Callers (both routes now use SSOT):
- lifecycle.rs::finalize_module() - Builder lifecycle route
- joinir_function_converter.rs::propagate_types() - JoinIR bridge route

Fail-fast guard (structural guarantee):
- PHI type inference is private step inside TypePropagationPipeline
- lifecycle.rs and joinir_function_converter.rs cannot call PhiTypeResolver directly
- Only public API: TypePropagationPipeline::run()
- Order drift is structurally impossible (private encapsulation)

Code reduction:
- ~500 lines of duplicate BinOp re-propagation logic removed
- 2 implementations consolidated into 1 SSOT

Files changed:
- New: src/mir/type_propagation/mod.rs
- New: src/mir/type_propagation/pipeline.rs (~300 lines)
- Modified: src/mir/mod.rs
- Modified: src/mir/builder/lifecycle.rs (~400 lines removed)
- Modified: src/mir/join_ir_vm_bridge/joinir_function_converter.rs (~100 lines removed)

Regression testing:
✅ Lifecycle route (VM backend): Phase 275 fixture
✅ JoinIR route (VM backend): loop_min_while.hako
✅ LLVM harness: Phase 275 fixture

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

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

docs/development/current/main/phases/phase-279/README.md

@@ -1,6 +1,6 @@
-# Phase 279 (planned): Type propagation pipeline SSOT unification
+# Phase 279 P0: Type propagation pipeline SSOT unification
 
-Status: planned / implementation
+Status: ✅ completed (2025-12-22)
 
 Goal: eliminate “two compiler pipelines” by making type propagation run through **one SSOT entry** with a fixed order across routes, so the same fixture cannot pass in one route and fail in another purely due to ordering drift.
 
@@ -18,8 +18,71 @@ SSOT references:
 
 Implementation guide:
 - `docs/development/current/main/phases/phase-279/P0-INSTRUCTIONS.md`
+- Claude Code instructions:
+  - `docs/development/current/main/phases/phase-279/P0-CLAUDE.md`
 
 Non-goals:
 - new language features (no Union/Any)
 - broad optimizer rewrite
 - adding new environment variables
+
+---
+
+## SSOT Entry Point
+
+**File**: `src/mir/type_propagation/pipeline.rs`
+
+This is the **single source of truth** for type propagation. All routes MUST call this pipeline.
+
+### Main Entry
+
+- `TypePropagationPipeline::run(function, value_types)` - SSOT entry point
+
+### Pipeline Steps (fixed order)
+
+1. **Copy propagation** (initial) - Propagate types through Copy chains
+2. **BinOp re-propagation** - Numeric promotion (Int+Float→Float, String+String→StringBox)
+3. **Copy propagation** (after promotion) - Propagate promoted types
+4. **PHI type inference** (private step) - Resolve PHI node types
+
+### Callers (exhaustive list)
+
+- `src/mir/builder/lifecycle.rs::finalize_module()` - Builder lifecycle route (AST → MIR)
+- `src/mir/join_ir_vm_bridge/joinir_function_converter.rs::propagate_types()` - JoinIR bridge route (JoinIR → MIR)
+
+### Consumer (read-only)
+
+- LLVM harness (Python `llvm_py/`) - Consumer only, no type propagation logic (best-effort forbidden)
+
+### Fail-Fast Guards
+
+**Structural guarantee** (Phase 279 P0):
+- PHI type inference is a **private step** inside TypePropagationPipeline
+- lifecycle.rs and joinir_function_converter.rs **cannot call PhiTypeResolver directly**
+- Only public API: `TypePropagationPipeline::run()`
+- Order drift is **structurally impossible** (private encapsulation)
+
+**Environment variables** (existing, reused):
+- `NYASH_PHI_GLOBAL_DEBUG=1` - PHI type inference debug output
+- `NYASH_BINOP_REPROP_DEBUG=1` - BinOp re-propagation debug output
+
+**No new environment variables** (Phase 279 P0 policy: prevent env var sprawl)
+
+---
+
+## Implementation Summary
+
+**Files changed** (6 total):
+- New: `src/mir/type_propagation/mod.rs` - Module definition
+- New: `src/mir/type_propagation/pipeline.rs` - SSOT pipeline implementation (~300 lines)
+- Modified: `src/mir/mod.rs` - Add type_propagation module export
+- Modified: `src/mir/builder/lifecycle.rs` - Replace with SSOT call (~400 lines removed)
+- Modified: `src/mir/join_ir_vm_bridge/joinir_function_converter.rs` - Replace with SSOT call (~100 lines removed)
+- Modified: `docs/development/current/main/phases/phase-279/README.md` - Document SSOT entry
+
+**Code reduction**:
+- Removed ~500 lines of duplicate BinOp re-propagation logic
+- Consolidated 2 implementations into 1 SSOT
+
+**Completion**:
+- `docs/development/current/main/phases/phase-279/P0-COMPLETION.md` - Completion record