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/phi.rs

@@ -88,7 +88,7 @@ impl MirBuilder {
                     if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
                         crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
                     }
-                    let merged = self.value_gen.next();
+                    let merged = self.next_value_id();
                     if let (Some(func), Some(cur_bb)) = (self.current_function.as_mut(), self.current_block) {
                         crate::mir::ssot::cf_common::insert_phi_at_head(func, cur_bb, merged, inputs);
                     } else {
@@ -123,7 +123,7 @@ impl MirBuilder {
         let assigned_var_else = else_ast_for_analysis
             .as_ref()
             .and_then(|a| crate::mir::phi_core::if_phi::extract_assigned_var(a));
-        let result_val = self.value_gen.next();
+        let result_val = self.next_value_id();
 
         // フェーズM: no_phi_mode分岐削除（常にPHI命令を使用）