refactor(sports): convert sports.py into a package (pure move)

Phase B1a of docs/SPORTS_UNIFICATION.md. src/base_classes/sports.py
becomes a package so the upcoming capability modules have a home and
diffs show their blast radius:

  sports/__init__.py   re-exports the public API
  sports/core.py       SportsCore
  sports/modes.py      SportsUpcoming / SportsRecent / SportsLive

No logic change: the 1515 class-body lines are byte-identical to the
original (verified by concatenating the two modules and diffing against
HEAD). Only module docstrings and the redistributed import blocks are
new. MRO and __abstractmethods__ are unchanged, and every existing
import site — including 'from src.base_classes.sports import SportsCore'
in the sport subclasses, the skin tests, and the characterization
suite — resolves through the package __init__.

One test edit was required: the characterization suite monkeypatched
'src.base_classes.sports.get_background_service', which is no longer a
module attribute on a package. Retargeted to
'src.base_classes.sports.core.get_background_service' — the module whose
globals SportsCore.__init__ actually resolves, so the patch is effective
exactly as before. No test logic or assertion changed.

Also adds docs/SPORTS_UNIFICATION.md: the architecture for the whole
B1-B5 sequence — how upgradability (guarded imports, capability probing,
frozen view-model keys, the sunset rule), reusability (promote only what
all nine copies share), and modularity (capabilities as opt-in mixins
rather than config branches, variants as named strategies, sport-unique
code as declared override points) are kept as three separate mechanisms.

Verified: characterization + skin 94 passed; the 10-file unit suite 338
passed; test/plugins 60 passed — all identical to pre-change counts.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FgbA8SMutQQpXkMG8LMmC4

test/test_sports_base_characterization.py

@@ -400,7 +400,7 @@ def build_manager(monkeypatch, tmp_path):
     monkeypatch.setattr(
         SportsCore, "_initialize_logo_dir", lambda self, configured: tmp_path)
     monkeypatch.setattr(
-        "src.base_classes.sports.get_background_service",
+        "src.base_classes.sports.core.get_background_service",
         lambda *args, **kwargs: MagicMock())
 
     def build(cls, schedule, **mode_cfg):