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.rs

@@ -76,7 +76,7 @@ impl super::MirBuilder {
                     if let Some(type_name) = Self::extract_string_literal(&m.arguments[0]) {
                         let obj_val = self.build_expression_impl(*m.object.clone())?;
                         let ty = Self::parse_type_name_to_mir(&type_name);
-                        let dst = self.value_gen.next();
+                        let dst = self.next_value_id();
                         let op = if m.method == "is" { crate::mir::TypeOpKind::Check } else { crate::mir::TypeOpKind::Cast };
                         self.emit_instruction(MirInstruction::TypeOp { dst, op, value: obj_val, ty })?;
                         return Ok(dst);
@@ -244,7 +244,7 @@ impl super::MirBuilder {
             } => self.build_new_expression(class.clone(), arguments.clone()),
 
             ASTNode::ArrayLiteral { elements, .. } => {
-                let arr_id = self.value_gen.next();
+                let arr_id = self.next_value_id();
                 self.emit_instruction(MirInstruction::NewBox {
                     dst: arr_id,
                     box_type: "ArrayBox".to_string(),
@@ -278,7 +278,7 @@ impl super::MirBuilder {
                 Ok(arr_id)
             }
             ASTNode::MapLiteral { entries, .. } => {
-                let map_id = self.value_gen.next();
+                let map_id = self.next_value_id();
                 self.emit_instruction(MirInstruction::NewBox {
                     dst: map_id,
                     box_type: "MapBox".to_string(),
@@ -409,7 +409,7 @@ impl super::MirBuilder {
         match class_hint.as_deref() {
             Some("ArrayBox") => {
                 let index_val = self.build_expression(index)?;
-                let dst = self.value_gen.next();
+                let dst = self.next_value_id();
                 self.emit_box_or_plugin_call(
                     Some(dst),
                     target_val,
@@ -422,7 +422,7 @@ impl super::MirBuilder {
             }
             Some("MapBox") => {
                 let index_val = self.build_expression(index)?;
-                let dst = self.value_gen.next();
+                let dst = self.next_value_id();
                 self.emit_box_or_plugin_call(
                     Some(dst),
                     target_val,