Step 10a + 10b: scaffold rcjav/ package, extract ID rules

Carves the first slice out of the monolithic rc-jav.py (now 2017
lines, was 2230). Two new modules:

  rcjav/model.py    FileEntry dataclass — the one shared shape that
                    every other submodule will need.
  rcjav/ids.py      Single source of truth for everything that
                    influences a FileEntry.jav_id: PRIMARY_ID_RE,
                    FALLBACK_ID_RE, COMPOUND_ID_RE, BUILTIN_PART_RES,
                    configure_part_patterns, detect_part,
                    detect_part_from_stem, part_key, extract_id,
                    normalize_id, describe_id_match, expand_range,
                    plus the supporting "private" regexes
                    (_BRACKET_ID_RE, _RESOLUTION_TAG_RE, etc.) that
                    other code in rc-jav.py still reads.

rcjav/__init__.py re-exports the public surface so future external
consumers can `from rcjav import extract_id` without caring which
submodule it lives in.

rc-jav.py drops the inline ID block and pulls everything from
rcjav.ids via a single import statement. PART_RES is intentionally
NOT imported — it's mutated by configure_part_patterns at runtime, so
a captured top-level reference would go stale. A small helper
`_current_part_res()` reads it dynamically via `_rcjav_ids.PART_RES`.

fixtures/run.py fix: synthesized importlib module name changed from
"rcjav" (which now collides with the real package directory) to
"rcjav_script". Also prepends ROOT to sys.path so rc-jav.py's
`from rcjav.model import …` resolves when run as
`python fixtures/run.py`.

Verified:
  - python rc-jav.py --help              → usage banner prints
  - 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

@@ -0,0 +1,24 @@
+"""rcjav — internal package split out of rc-jav.py.
+
+This file re-exports the names that external callers (tests, fixtures
+runner, native messaging host, in-tree code in rc-jav.py) expect to
+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.ids import (  # noqa: F401
+    PRIMARY_ID_RE,
+    FALLBACK_ID_RE,
+    COMPOUND_ID_RE,
+    RANGE_RE,
+    BUILTIN_PART_RES,
+    PART_RES,
+    configure_part_patterns,
+    detect_part,
+    detect_part_from_stem,
+    part_key,
+    extract_id,
+    normalize_id,
+    describe_id_match,
+    expand_range,
+)