fix(common): address review on sports_helpers and the hardware-free test

- SportsHelpersMixin docstring and CHANGELOG: constructor-free, but it keeps
  lazy state on its host (_reset_dwell_on_reentry, _next_switch_index).
- test_common_is_hardware_free: the runtime check now filters every
  FORBIDDEN package, src.plugin_system included; the AST scan resolves
  relative imports against src.common, so `from .. import plugin_system`
  and `from ..plugin_system import x` are caught. Guard tests for both.
- Parity skip reason names the CI guard that runs the same comparison:
  ledmatrix-plugins scripts/check_sports_helpers_parity.py (#495).
- _odds_color: line-level pylint disable for a not-callable false positive
  (getter is None-checked); the AST is unchanged, parity still passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -29,7 +29,8 @@ this):
   `_setting_int`, `_reset_dwell_on_reentry`, `_next_switch_index`,
   `_spread_weighted_order`, `_odds_color`, `_upcoming_date_and_time_text` under
   the plugins' names and signatures, plus the `_favorite_key` override point.
-  No `__init__` and no state; the host contract is in the module docstring.
+  Constructor-free; keeps lazy state on its host (see the module docstring,
+  which also gives the host contract).
   A new module rather than more methods on `sports_shared`: a plugin that
   deletes a copy and leans on an older module having grown the method fails at
   runtime with `AttributeError`, which no load-time check sees, while a missing