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/utils.rs

@@ -25,6 +25,19 @@ pub(super) fn builder_debug_log(msg: &str) {
 }
 
 impl super::MirBuilder {
+    // ---- Value ID allocation (function-local or module-global) ----
+    /// Allocate a new ValueId in the appropriate context
+    /// - Inside function: uses function-local allocator
+    /// - Outside function: uses module-global allocator
+    #[inline]
+    pub(super) fn next_value_id(&mut self) -> super::ValueId {
+        if let Some(ref mut f) = self.current_function {
+            f.next_value_id()  // Function context
+        } else {
+            self.value_gen.next()  // Module context
+        }
+    }
+
     // ---- LocalSSA convenience (readability helpers) ----
     #[allow(dead_code)]
     #[inline]
@@ -72,7 +85,7 @@ impl super::MirBuilder {
                 for name in names.iter() {
                     if !name.starts_with("__pin$") { continue; }
                     if let Some(&src) = self.variable_map.get(name) {
-                        let dst = self.value_gen.next();
+                        let dst = self.next_value_id();
                         self.emit_instruction(super::MirInstruction::Copy { dst, src })?;
                         crate::mir::builder::metadata::propagate::propagate(self, src, dst);
                         self.variable_map.insert(name.clone(), dst);
@@ -201,7 +214,7 @@ impl super::MirBuilder {
         value: super::ValueId,
         expected_type: String,
     ) -> Result<super::ValueId, String> {
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
         self.emit_instruction(super::MirInstruction::TypeOp {
             dst,
             op: TypeOpKind::Check,
@@ -217,7 +230,7 @@ impl super::MirBuilder {
         value: super::ValueId,
         target_type: super::MirType,
     ) -> Result<super::ValueId, String> {
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
         self.emit_instruction(super::MirInstruction::TypeOp {
             dst,
             op: TypeOpKind::Cast,
@@ -235,7 +248,7 @@ impl super::MirBuilder {
         if crate::config::env::mir_core13_pure() {
             return Ok(box_val);
         }
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
         self.emit_instruction(super::MirInstruction::WeakRef {
             dst,
             op: WeakRefOp::New,
@@ -252,7 +265,7 @@ impl super::MirBuilder {
         if crate::config::env::mir_core13_pure() {
             return Ok(weak_ref);
         }
-        let dst = self.value_gen.next();
+        let dst = self.next_value_id();
         self.emit_instruction(super::MirInstruction::WeakRef {
             dst,
             op: WeakRefOp::Load,
@@ -282,7 +295,12 @@ impl super::MirBuilder {
     pub(crate) fn pin_to_slot(&mut self, v: super::ValueId, hint: &str) -> Result<super::ValueId, String> {
         self.temp_slot_counter = self.temp_slot_counter.wrapping_add(1);
         let slot_name = format!("__pin${}${}", self.temp_slot_counter, hint);
-        let dst = self.value_gen.next();
+        // Phase 25.1b: Use function-local ID allocator to avoid SSA verification failures
+        let dst = if let Some(ref mut f) = self.current_function {
+            f.next_value_id()  // Function context: use local ID
+        } else {
+            self.value_gen.next()  // Module context: use global ID
+        };
         self.emit_instruction(super::MirInstruction::Copy { dst, src: v })?;
         if super::utils::builder_debug_enabled() || std::env::var("NYASH_PIN_TRACE").ok().as_deref() == Some("1") {
             super::utils::builder_debug_log(&format!("pin slot={} src={} dst={}", slot_name, v.0, dst.0));
@@ -295,7 +313,12 @@ impl super::MirBuilder {
 
     /// Ensure a value has a local definition in the current block by inserting a Copy.
     pub(crate) fn materialize_local(&mut self, v: super::ValueId) -> Result<super::ValueId, String> {
-        let dst = self.value_gen.next();
+        // Phase 25.1b: Use function-local ID allocator to avoid SSA verification failures
+        let dst = if let Some(ref mut f) = self.current_function {
+            f.next_value_id()  // Function context: use local ID
+        } else {
+            self.value_gen.next()  // Module context: use global ID
+        };
         self.emit_instruction(super::MirInstruction::Copy { dst, src: v })?;
         // Propagate metadata (type/origin) from source to the new local copy
         crate::mir::builder::metadata::propagate::propagate(self, v, dst);