refactor(#27): IMP-27 K5 catalog loader + _get_block_by_id cleanup

Consolidate three duplicated catalog readers and two _get_block_by_id
implementations behind a single shared module (src/catalog.py) that owns
file-read + mtime cache. All caller signatures and return contracts
remain byte-identical.

Units:
- u1 NEW src/catalog.py (76 lines): load_root_catalog / load_blocks /
  get_block_by_id / get_catalog_mtime as the sole file-read +
  mtime-cache owner.
- u2 src/block_reference.py: _load_catalog delegates to load_blocks
  (list[dict] preserved); _get_block_by_id (no-arg) delegates to
  catalog.get_block_by_id. Module-level _catalog_cache removed.
- u3 src/block_selector.py: load_catalog delegates to load_root_catalog
  (root dict preserved); _get_block_by_id (catalog-injected sig
  preserved) delegates to catalog.get_block_by_id. Module-level
  _catalog_cache / _catalog_mtime / CATALOG_PATH removed.
- u4 src/renderer.py: _load_catalog_map and
  _load_catalog_map_with_variants consume catalog.load_blocks; renderer
  projection caches kept local but keyed via
  catalog.get_catalog_mtime(). Per-projection invalidation keys
  (_CATALOG_MAP_MTIME / _CATALOG_VARIANT_MAP_MTIME) introduced. import
  yaml, CATALOG_PATH, legacy _CATALOG_MTIME removed.
- tests NEW tests/test_catalog_shared_loader.py (421 lines, 23 cases):
  shared loader + 3 wrappers covering single file-read, contract
  preservation, signature preservation, shared cache, private state
  absence, mtime invalidation propagation to renderer projections.

Verification:
- pytest tests/test_catalog_shared_loader.py -v: 23/23 PASS in 0.13s.
- pytest tests/ -q --ignore=tests/matching: 365/365 PASS in 38.10s.
- src/fit_verifier.py, src/space_allocator.py, src/pipeline.py and
  templates/catalog.yaml unchanged (git diff empty).

Out of scope:
- catalog.yaml schema/path unchanged.
- Catalog direct-read call sites in fit_verifier / space_allocator /
  pipeline left for a separate follow-up axis.
- Phase Z 22-step runtime, frame_selection, light_edit/restructure
  flows untouched.

Refs: IMP-27 (gitea #27), INSIGHT-MAP §5 K5, PHASE-Q-AUDIT §2.10

src/block_selector.py

@@ -5,24 +5,18 @@ AI에게 불가능한 선택지를 주지 않는다 (Beautiful.ai 원칙).
 
 주요 함수:
 - select_block_candidates(): topic + 컨테이너 → 물리적으로 가능한 후보 2-4개
-- load_catalog(): catalog.yaml 로딩 + 캐싱
+- load_catalog(): catalog.yaml 로딩 + 캐싱 (IMP-27: src.catalog 공유 로더 위임)
 """
 from __future__ import annotations
 
 import logging
-from pathlib import Path
 from typing import Any
 
-import yaml
-
+from src import catalog as _catalog_mod
 from src.space_allocator import ContainerSpec, HEIGHT_COST_ORDER
 
 logger = logging.getLogger(__name__)
 
-CATALOG_PATH = Path("templates/catalog.yaml")
-_catalog_cache: dict | None = None
-_catalog_mtime: float = 0.0
-
 
 # ──────────────────────────────────────
 # relation_type → 블록 카테고리 매핑 (Napkin.ai 방식)
@@ -52,35 +46,16 @@ BLOCKS_FORCING_FORMAT_CHANGE = {
 
 
 # ──────────────────────────────────────
-# catalog.yaml 로딩 (mtime 캐시)
+# catalog.yaml 로딩 (IMP-27: src.catalog 공유 로더 위임)
 # ──────────────────────────────────────
 def load_catalog() -> dict:
-    """catalog.yaml을 로딩한다. mtime 기반 캐싱."""
-    global _catalog_cache, _catalog_mtime
-
-    if not CATALOG_PATH.exists():
-        logger.error(f"catalog.yaml 미발견: {CATALOG_PATH}")
-        return {"blocks": []}
-
-    current_mtime = CATALOG_PATH.stat().st_mtime
-    if _catalog_cache is not None and current_mtime == _catalog_mtime:
-        return _catalog_cache
-
-    with open(CATALOG_PATH, encoding="utf-8") as f:
-        _catalog_cache = yaml.safe_load(f)
-    _catalog_mtime = current_mtime
-
-    block_count = len(_catalog_cache.get("blocks", []))
-    logger.info(f"[Q-2] catalog.yaml 로딩: {block_count}개 블록")
-    return _catalog_cache
+    """catalog.yaml root dict (IMP-27: shared loader delegation)."""
+    return _catalog_mod.load_root_catalog()
 
 
 def _get_block_by_id(block_id: str, catalog: dict) -> dict | None:
-    """catalog에서 블록 ID로 검색."""
-    for block in catalog.get("blocks", []):
-        if block.get("id") == block_id:
-            return block
-    return None
+    """catalog-injected 블록 ID 조회 (IMP-27: shared loader delegation)."""
+    return _catalog_mod.get_block_by_id(block_id, catalog)
 
 
 # ──────────────────────────────────────