fix(mir): fix else block scope bug - PHI materialization order

Root Cause:
- Else blocks were not propagating variable assignments to outer scope
- Bug 1 (if_form.rs): PHI materialization happened before variable_map reset,
  causing PHI nodes to be lost
- Bug 2 (phi.rs): Variable merge didn't check if else branch modified variables

Changes:
- src/mir/builder/if_form.rs:93-127
  - Reordered: reset variable_map BEFORE materializing PHI nodes
  - Now matches then-branch pattern (reset → materialize → execute)
  - Applied to both "else" and "no else" branches for consistency
- src/mir/builder/phi.rs:137-154
  - Added else_modified_var check to detect variable modifications
  - Use modified value from else_var_map_end_opt when available
  - Fall back to pre-if value only when truly not modified

Test Results:
✅ Simple block: { x=42 } → 42
✅ If block: if 1 { x=42 } → 42
✅ Else block: if 0 { x=99 } else { x=42 } → 42 (FIXED!)
✅ Stage-B body extraction: "return 42" correctly extracted (was null)

Impact:
- Else block variable assignments now work correctly
- Stage-B compiler body extraction restored
- Selfhost builder path can now function
- Foundation for Phase 21.x progress

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

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

docs/reference/language/README.md

@@ -16,6 +16,9 @@ Statement separation and semicolons
 Imports and namespaces
 - See: reference/language/using.md — `using` syntax, runner resolution, and style guidance.
 
+Variables and scope
+- See: reference/language/variables-and-scope.md — Block-scoped locals, assignment resolution, and strong/weak reference guidance.
+
 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`).