fix(mir/builder): use function-local ValueId throughout MIR builder

Phase 25.1b: Complete SSA fix - eliminate all global ValueId usage in function contexts.

Root cause: ~75 locations throughout MIR builder were using global value
generator (self.value_gen.next()) instead of function-local allocator
(f.next_value_id()), causing SSA verification failures and runtime
"use of undefined value" errors.

Solution:
- Added next_value_id() helper that automatically chooses correct allocator
- Fixed 19 files with ~75 occurrences of ValueId allocation
- All function-context allocations now use function-local IDs

Files modified:
- src/mir/builder/utils.rs: Added next_value_id() helper, fixed 8 locations
- src/mir/builder/builder_calls.rs: 17 fixes
- src/mir/builder/ops.rs: 8 fixes
- src/mir/builder/stmts.rs: 7 fixes
- src/mir/builder/emission/constant.rs: 6 fixes
- src/mir/builder/rewrite/*.rs: 10 fixes
- + 13 other files

Verification:
- cargo build --release: SUCCESS
- Simple tests with NYASH_VM_VERIFY_MIR=1: Zero undefined errors
- Multi-parameter static methods: All working

Known remaining: ValueId(22) in Stage-B (separate issue to investigate)

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

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

src/runner/child_env.rs

@@ -52,3 +52,20 @@ pub fn apply_core_wrapper_env(cmd: &mut std::process::Command) {
         cmd.env("NYASH_PARSER_ALLOW_SEMICOLON", val);
     }
 }
+
+/// Apply environment for selfhost/Ny compiler processes (ParserBox/EmitterBox/MirBuilderBox).
+/// - Inherits core restrictions from apply_core_wrapper_env()
+/// - Re-enables `using` file resolution for module loading (lang.compiler.parser.box, etc.)
+/// - Keeps plugin/toml restrictions to avoid side effects
+pub fn apply_selfhost_compiler_env(cmd: &mut std::process::Command) {
+    // Phase 25.1b: Start with core wrapper restrictions
+    apply_core_wrapper_env(cmd);
+
+    // Phase 25.1b: Re-enable file-based using for selfhost compiler module resolution
+    // Selfhost compiler uses `using lang.compiler.parser.box`, which requires file resolution
+    // (nyash.toml [modules] mapping is primary, but file fallback ensures robustness)
+    cmd.env("HAKO_ALLOW_USING_FILE", "1");
+
+    // Note: NYASH_USING_AST stays 0 (AST-based using not needed for basic module resolution)
+    // Note: NYASH_DISABLE_PLUGINS stays 1 (avoid plugin side effects in nested compilation)
+}