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_peek.rs

@@ -14,7 +14,7 @@ impl super::MirBuilder {
 
         // Prepare merge and result
         let merge_block: BasicBlockId = self.block_gen.next();
-        let result_val = self.value_gen.next();
+        let result_val = self.next_value_id();
         let mut phi_inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();
 
         // Create dispatch block where we start comparing arms
@@ -76,7 +76,7 @@ impl super::MirBuilder {
                 LiteralValue::Null => crate::mir::builder::emission::constant::emit_null(self),
                 LiteralValue::Void => crate::mir::builder::emission::constant::emit_void(self),
             };
-            let cond_id = self.value_gen.next();
+            let cond_id = self.next_value_id();
             crate::mir::builder::emission::compare::emit_to(self, cond_id, super::CompareOp::Eq, scr_val, lit_id)?;
             crate::mir::builder::emission::branch::emit_conditional(self, cond_id, then_block, else_target)?;