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/modes/common_util/selfhost/child.rs

@@ -17,8 +17,8 @@ pub fn run_ny_program_capture_json(
 ) -> Option<String> {
     use std::process::Command;
     let mut cmd = Command::new(exe);
-    // Apply consistent child env to avoid plugin/using drift
-    crate::runner::child_env::apply_core_wrapper_env(&mut cmd);
+    // Phase 25.1b: Use selfhost compiler env (enables using/file resolution for compiler.hako)
+    crate::runner::child_env::apply_selfhost_compiler_env(&mut cmd);
     cmd.arg("--backend").arg("vm").arg(program);
     for a in extra_args {
         cmd.arg(a);