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>

tools/selfhost/examples/hako_llvm_selfhost_driver.hako

@@ -5,6 +5,7 @@
 // Prints the exe path to stdout.
 
 using selfhost.vm.helpers.mir_call_v1_handler as MirCallV1HandlerBox // not required but keeps linker alive
+using selfhost.shared.host_bridge.codegen_bridge as CodegenBridgeBox
 
 static box Main {
   method main(args) {
@@ -14,11 +15,11 @@ static box Main {
     if exe_out == null { exe_out = "/tmp/hako_selfhost_exe" }
     // emit object
     local a = new ArrayBox(); a.push(j)
-    local obj = hostbridge.extern_invoke("env.codegen", "emit_object", a)
+    local obj = CodegenBridgeBox.emit_object_args(a)
     if obj == null { print("[ERR] emit_object failed"); return 2 }
     // link exe
     local b = new ArrayBox(); b.push(obj); b.push(exe_out)
-    local exe = hostbridge.extern_invoke("env.codegen", "link_object", b)
+    local exe = CodegenBridgeBox.link_object_args(b)
     if exe == null { print("[ERR] link_object failed"); return 3 }
     print("" + exe)
     return 0