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/runtime/extern_registry.rs

@@ -147,6 +147,13 @@ static EXTERNS: Lazy<Vec<ExternSpec>> = Lazy::new(|| {
             max_arity: 255,
             slot: Some(50),
         },
+        ExternSpec {
+            iface: "env.box_introspect",
+            method: "kind",
+            min_arity: 1,
+            max_arity: 1,
+            slot: Some(51),
+        },
         // basic env access
         ExternSpec {
             iface: "env",

src/runtime/plugin_loader_v2/enabled/mod.rs

@@ -14,6 +14,7 @@ pub use types::{
     construct_plugin_box, make_plugin_box_v2, NyashTypeBoxFfi, PluginBoxMetadata, PluginBoxV2,
     PluginHandleInner,
 };
+pub use extern_functions::handle_box_introspect;
 
 pub fn metadata_for_type_id(type_id: u32) -> Option<PluginBoxMetadata> {
     let loader = get_global_loader_v2();