Step 10c: extract cache I/O into rcjav/cache.py

Pulls CACHE_PATH, CACHE_VERSION, CACHE_STALE_HOURS, load_cache,
save_cache, cache_age_hours, and fmt_age out of rc-jav.py and into a
new self-contained module. No behavior change.

rc-jav.py: 2019 → 1972 lines.

The new module's `CACHE_PATH = Path(__file__).resolve().parents[1] /
"cache.json"` keeps the file at the repo root next to rc-jav.py (one
directory above the package), matching the legacy `Path(__file__).
resolve().parent / "cache.json"` location.

rcjav/__init__.py now re-exports the cache public surface alongside
the model and ids surface.

Verified:
  - python rc-jav.py --help              → ok
  - python fixtures/run.py               → 17/17 cases pass
  - python -m unittest tests.test_rules  → 5/5 OK

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

rcjav/__init__.py

@@ -6,6 +6,15 @@ find at the top level. Adding a new submodule does not change the
 public surface — only this file does.
 """
 from rcjav.model import FileEntry  # noqa: F401
+from rcjav.cache import (  # noqa: F401
+    CACHE_PATH,
+    CACHE_VERSION,
+    CACHE_STALE_HOURS,
+    load_cache,
+    save_cache,
+    cache_age_hours,
+    fmt_age,
+)
 from rcjav.ids import (  # noqa: F401
     PRIMARY_ID_RE,
     FALLBACK_ID_RE,