refactor: modularize shape_guard.rs and separate canonicalizer tests

shape_guard.rs (1401 lines → 6 files):
- pattern2.rs: Break-loop patterns (132 lines)
- pattern3.rs: If-sum patterns (67 lines)
- pattern4.rs: Continue patterns (152 lines)
- selfhost.rs: Selfhost-specific detectors (245 lines)
- utils.rs: Shared helpers (33 lines)
- mod.rs: API coordination & tests (776 lines)

canonicalizer.rs (1863 lines → 433 lines):
- Separated 1430 lines of tests to canonicalizer_tests.rs

Benefits:
- Improved maintainability (smaller, focused files)
- Clear separation of concerns (pattern families)
- Preserved API compatibility (re-exports)
- No new warnings

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/mir/loop_canonicalizer/mod.rs

@@ -39,6 +39,9 @@ mod capability_guard;
 mod pattern_recognizer;
 mod skeleton_types;
 
+#[cfg(test)]
+mod canonicalizer_tests;
+
 // ============================================================================
 // Public Re-exports
 // ============================================================================