refactor: modernize batch — @override, single stop signal, struct cleanups

[tcoratger]

Summary

Five modernization items from the audit, bundled. Net +193 / −239 across 13 files.

Module	Change
subspecs/koalabear/field.py	@override added to is_fixed_size, get_byte_length, serialize, deserialize so the SSZType contract is visible at the method site.
subspecs/networking/client/event_source/live.py + subspecs/networking/gossipsub/behavior.py	After PR #743 both files carried _running (bool) and _stop_event (asyncio.Event). The event already exposes is_set/wait/set/clear, which covers every read and write the bool was doing. Dropped the bool; not self._stop_event.is_set() is the "running" predicate. The default factory stays a vanilla asyncio.Event; each file's __post_init__ forces it to set (= stopped) so fresh instances start stopped until dial/listen/start clears it. Updated __main__.py, node_runner.py, and 6 affected unit tests.
subspecs/storage/namespaces.py + sqlite.py	Five @dataclass(frozen=True, slots=True) namespace classes whose only contents were string defaults collapsed to plain Final[str] module constants (BLOCKS_TABLE_NAME, BLOCKS_CREATE_TABLE, BLOCKS_CREATE_INDEX, ...). 30+ attribute accesses in sqlite.py and its import block rewritten.
subspecs/xmss/tweak_hash.py	TreeTweak and ChainTweak were Pydantic StrictBaseModel instances used only as packing structs feeding the match tweak block. Converted both to typing.NamedTuple — the match patterns keep working since NamedTuple auto-populates __match_args__. The from __future__ import annotations line that the audit flagged as R1 (banned in Pydantic-defining files) goes away with the conversion; TweakHasher itself stays a StrictBaseModel.
subspecs/xmss/constants.py	TWEAK_PREFIX_CHAIN/TREE/MESSAGE were Fp(value=0x00..0x02) wrappers whose every consumer immediately did .value. Inlined as plain Final[int]; dropped the unused Fp import and rewrote three call sites plus one test.

Test plan

• ruff check — clean.
• ruff format --check — clean.
• ty check — clean (it caught 12 stale _running references in __main__.py and tests during iteration; all migrated).
• pytest over koalabear, event_source, gossipsub publish, storage, xmss — 205 tests pass.
• fill tests/consensus/lstar/fc/test_tick_system.py::test_tick_interval_progression_through_full_slot — 1 passed in 9.16s (consensus fixture pipeline still threads through after the lifecycle signal change).

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