refactor(plan): Phase 286C-4 Step 1 - plan_helpers module

Create helper functions to support plan_rewrites() extraction:
- build_local_block_map(): Build block ID mapping for a function
- sync_spans(): Synchronize instruction spans after rewriting

These pure functions will be used by both the current monolithic
merge_and_rewrite() and the new plan_rewrites() function.

Progress: Step 1/4 (helpers) complete

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

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

docs/reference/language/EBNF.md

@@ -17,7 +17,21 @@ logic     := compare (('&&' | '||') compare)*
 compare   := sum (( '==' | '!=' | '<' | '>' | '<=' | '>=' ) sum)?
 sum       := term (('+' | '-') term)*
 term      := unary (('*' | '/') unary)*
-unary     := ('-' | '!' | 'not' | '~' | 'weak') unary | factor
+unary     := ( '-' | '!' | 'not' | '~' ) unary
+           | weak_unary
+           | factor
+
+; Phase 285W-Syntax-0.1: `weak(<expr>)` is invalid. The operand must not be a grouped
+; expression starting with `(`. (Write `weak x`, not `weak(x)`.)
+weak_unary := 'weak' unary_no_group
+unary_no_group := ( '-' | '!' | 'not' | '~' ) unary_no_group
+               | INT
+               | STRING
+               | IDENT call_tail*
+               | 'new' IDENT '(' args? ')'
+               | '[' args? ']'           ; Array literal (Stage‑1 sugar, gated)
+               | '{' map_entries? '}'    ; Map literal (Stage‑2 sugar, gated)
+               | match_expr              ; Pattern matching (replaces legacy peek)
 
 factor    := INT
            | STRING

docs/reference/language/lifecycle.md

@@ -304,7 +304,7 @@ This section documents current backend reality so we can detect drift as bugs.
 
 | Feature | VM | LLVM | WASM |
 |---------|-----|------|------|
-| WeakRef (`weak <expr>`, `weak_to_strong()`) | ✅ | ❌ unsupported (285LLVM-1) | ❌ unsupported |
+| WeakRef (`weak <expr>`, `weak_to_strong()`) | ✅ | ✅ LLVM harness (Phase 285LLVM-1.4) | ❌ unsupported |
 | Leak Report (`NYASH_LEAK_LOG`) | ✅ | ⚠️ Parent process roots only (285LLVM-0) | ❌ |
 
 **LLVM Leak Report の制限** (Phase 285LLVM-0):
@@ -316,7 +316,7 @@ This section documents current backend reality so we can detect drift as bugs.
 ### Notes
 
 - **Block-scoped locals** are the language model (`local` drops at `}`), but the *observable* effects depend on where the last strong reference is held.
-- **WeakRef** (Phase 285A0): VM backend fully supports `weak <expr>` and `weak_to_strong()`. LLVM harness support is planned for Phase 285LLVM-1.
+- **WeakRef** (Phase 285A0+): VM backend fully supports `weak <expr>` and `weak_to_strong()`. LLVM harness also supports this surface as of Phase 285LLVM-1.4.
 - **WASM backend** currently treats MIR `WeakNew/WeakLoad` as plain copies (weak behaves like strong). This does not satisfy the SSOT weak semantics yet (see also: `docs/guides/wasm-guide/planning/unsupported_features.md`).
 - **Leak Report** (Phase 285): `NYASH_LEAK_LOG={1|2}` prints exit-time diagnostics showing global roots still held (modules, host_handles, plugin_boxes). See `docs/reference/environment-variables.md`.
 - Conformance gaps (any backend differences from this document) must be treated as bugs and tracked explicitly; do not "paper over" differences by changing this SSOT without a decision.

docs/reference/language/quick-reference.md

@@ -26,7 +26,7 @@ Display & Conversion
 - JSONシリアライズ: `toJson(x)`（文字列）/ `toJsonNode(x)`（構造）
 
 Operators (precedence high→low)
-- Unary: `! ~ -`
+- Unary: `! ~ - weak` (`weak <expr>` produces a `WeakRef`; `weak(expr)` is invalid)
 - Multiplicative: `* / %`
 - Additive: `+ -`
 - Compare: `== != < <= > >=`