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/backend/mir_interpreter/handlers/externals.rs

@@ -188,6 +188,14 @@ impl MirInterpreter {
                 self.write_result(dst, VMValue::String(exe.to_string_lossy().into_owned()));
                 Ok(())
             }
+            ("env.box_introspect", "kind") => {
+                // Route env.box_introspect.kind to extern provider (plugin_loader_v2)
+                let ret = self
+                    .extern_provider_dispatch("env.box_introspect.kind", args)
+                    .unwrap_or(Ok(VMValue::Void))?;
+                self.write_result(dst, ret);
+                Ok(())
+            }
             ("hostbridge", "extern_invoke") => {
                 if let Some(res) = self.extern_provider_dispatch("hostbridge.extern_invoke", args) {
                     match res {