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