refactor(mir): Extract TypeFacts Box from ops.rs (Phase 136 follow-up 1/3)

## Summary
Extracts type inference logic from ops.rs into independent TypeFacts Box,
improving testability and responsibility separation.

## Changes
- New file: src/mir/builder/type_facts.rs (TypeFactsBox + OperandTypeClass)
- Modified: src/mir/builder/ops.rs - Use TypeFacts Box instead of method
- Modified: src/mir/builder.rs - Add type_facts module

## Effect
- Type inference logic is now independently testable
- ops.rs responsibility simplified to "operator lowering only"
- Enables future sharing with lifecycle.rs:repropagate_binop_types()

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder.rs

@@ -52,6 +52,7 @@ mod router; // RouterPolicyBox（Unified vs BoxCall）
 mod schedule; // BlockScheduleBox（物理順序: PHI→materialize→body）
 mod ssa; // LocalSSA helpers (in-block materialization)
 mod stmts;
+mod type_facts; // Phase 136 follow-up: Type inference facts box
 pub(crate) mod type_registry;
 mod types; // types::annotation / inference（型注釈/推論の箱: 推論は後段）
 mod utils;