feat(ttp): E.3.7 RuleEngine — evaluate + atomic-swap watch_store

Implements the rule engine body left empty at contract phase: evaluate()
dispatches by source_kind through self._by_kind, runs the rule's match
spec against event.payload, and emits one TTPTag per emits entry.
watch_store() loads the initial corpus from RuleStore.load_compiled,
then drains subscribe_changes, applying definition changes via
single-statement dict assignment (atomic swap, GIL-atomic to readers)
and state changes via NamedTuple._replace on the existing CompiledRule.

Why: with the FS + DB stores in place (E.3.5/E.3.6), the engine is the
last piece of the rule plane. Lifters (E.3.9–E.3.13) consume the
engine; the worker bootstrap (E.3.14) wires watch_store into the
asyncio event loop. After this commit a CompositeTagger constructed
with a RuleEngine + a populated rules dir will produce real tags.

Notes:
- CompiledRule.emits extended to 4-tuple
  (technique_id, sub_technique_id, tactic, confidence). Tactic + confidence
  ride per-emit so a single rule can carry multiple precision targets
  (the "one event maps to many techniques" property). Compile helpers in
  both backends extract them from the YAML emits dict; missing tactic
  or confidence is a deploy-time error.
- v0 match operator is "pattern" (regex). The field defaults per
  source_kind (command_text / raw_url / subject / verdict / …) and is
  overridable via match.field. Future ops (contains, equals, in_set)
  extend _match_event without touching the engine surface.
- Confidence model: rules with state="clipped" + confidence_max set
  cap the per-emit confidence downward; clipped is a soft suppress, not
  a hard skip. Disabled rules are skipped wholly; expires_at past is
  re-checked at evaluate as defense-in-depth (the store auto-reverts,
  but a racing read between expiry and revert must not fire the rule).
- _span(name, **attrs) helper in engine + both stores short-circuits on
  decnet.telemetry._ENABLED — matches the project's @traced /
  wrap_repository zero-overhead-when-disabled pattern instead of relying
  solely on the no-op tracer indirection.
- Late-bound tracer (telemetry.get_tracer called per-span, not at
  module load) so test_tracing's monkeypatch reaches the production
  code path.

xfails flipped: tests/ttp/test_rule_engine.py multi-emit fan-out +
rule_version-collision-via-engine; tests/ttp/test_multi_mapping.py
N×M engine fan-out + idempotent replay; tests/ttp/test_tracing.py
ttp.eval span hierarchy + ttp.rule.fire span attributes.

Tests: 214 passed, 19 xfailed (gated on E.3.8 lifters / rule pack /
worker bootstrap).
mypy: clean on prod code; pre-existing test-stub arg-type warnings
unchanged.

tests/ttp/store/test_database.py

@@ -127,7 +127,7 @@ async def test_filesystem_to_db_sync_populates_ttp_rule(
         name="brute force ssh",
         applies_to=frozenset({"command"}),
         match_spec={"pattern": "hydra"},
-        emits=(("T1110", None),),
+        emits=(("T1110", None, "TA0006", 0.85),),
         evidence_fields=("matched_tokens",),
         state=RuleState(),
     )
@@ -151,7 +151,7 @@ async def test_filesystem_to_db_sync_populates_ttp_rule(
     loaded = await db_store.load_compiled()
     assert len(loaded) == 1
     assert loaded[0].rule_id == "R0001"
-    assert loaded[0].emits == (("T1110", None),)
+    assert loaded[0].emits == (("T1110", None, "TA0006", 0.85),)
 
 
 @pytest.mark.skipif(
@@ -186,7 +186,9 @@ applies_to: [command]
 match:
   pattern: 'whoami'
 emits:
-  - technique_id: T1033
+  - tactic: TA0007
+    technique_id: T1033
+    confidence: 0.85
 """,
                 encoding="utf-8",
             )