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/mir/builder/method_call_handlers.rs

@@ -24,7 +24,7 @@ impl MirBuilder {
 
         // Compose lowered function name: BoxName.method/N
         let func_name = format!("{}.{}/{}", box_name, method, arg_values.len());
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
 
         if std::env::var("NYASH_STATIC_CALL_TRACE").ok().as_deref() == Some("1") {
             eprintln!("[builder] static-call {}", func_name);
@@ -43,7 +43,7 @@ impl MirBuilder {
         type_name: &str,
     ) -> Result<ValueId, String> {
         let mir_ty = Self::parse_type_name_to_mir(type_name);
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
         let op = if method == "is" {
             TypeOpKind::Check
         } else {
@@ -99,7 +99,7 @@ impl MirBuilder {
 
         // Receiver class hintは emit_unified_call 側で起源/型から判断する（重複回避）
         // 統一経路: emit_unified_call に委譲（RouterPolicy と rewrite::* で安定化）
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
         self.emit_unified_call(
             Some(dst),
             CallTarget::Method { box_type: None, method, receiver: object_value },