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/exprs_call.rs

@@ -19,7 +19,7 @@ impl super::MirBuilder {
 
         if use_unified {
             // New unified path - use emit_unified_call with Value target
-            let dst = self.value_gen.next();
+            let dst = self.next_value_id();
             self.emit_unified_call(
                 Some(dst),
                 super::builder_calls::CallTarget::Value(callee_id),
@@ -28,7 +28,7 @@ impl super::MirBuilder {
             Ok(dst)
         } else {
             // Unified-off path: still encode callee as Value to avoid by-name resolution
-            let dst = self.value_gen.next();
+            let dst = self.next_value_id();
             self.emit_instruction(super::MirInstruction::Call {
                 dst: Some(dst),
                 func: callee_id,