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/schedule/block.rs

@@ -12,7 +12,7 @@ impl BlockScheduleBox {
             if let Some(&cached) = builder.schedule_mat_map.get(&(bb, src)) {
                 return Ok(cached);
             }
-            let dst = builder.value_gen.next();
+            let dst = builder.next_value_id();
             builder.insert_copy_after_phis(dst, src)?;
             builder.schedule_mat_map.insert((bb, src), dst);
             return Ok(dst);
@@ -25,7 +25,7 @@ impl BlockScheduleBox {
     pub fn emit_before_call_copy(builder: &mut MirBuilder, src: ValueId) -> Result<ValueId, String> {
         // Prefer to reuse the after-phis materialized id for this src in this block
         let base = Self::ensure_after_phis_copy(builder, src)?;
-        let dst = builder.value_gen.next();
+        let dst = builder.next_value_id();
         builder.emit_instruction(MirInstruction::Copy { dst, src: base })?;
         // Propagate metadata to keep dst consistent with base
         crate::mir::builder::metadata::propagate::propagate(builder, base, dst);