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>

lang/src/vm/boxes/mir_call_v1_handler.hako

@@ -322,6 +322,13 @@ static box MirCallV1HandlerBox {
         if dstp != null { regs.setField(StringHelpers.int_to_str(dstp), "") }
         return
       }
+      if name == "env.box_introspect.kind" {
+        local dstp = JsonFragBox.get_int(seg, "dst")
+        local aval = null; if arg0id >= 0 { aval = regs.getField(StringHelpers.int_to_str(arg0id)) }
+        HakoruneExternProviderBox.get(name, aval)
+        if dstp != null { regs.setField(StringHelpers.int_to_str(dstp), "") }
+        return
+      }
     }
     if name == "env.console.log" || name == "nyash.console.log" ||
        name == "env.console.warn" || name == "nyash.console.warn" ||