Adds DigestAccumulator tracking ACTIVE NOW and SINCE LAST DIGEST
state per toggle. Replaces StubDigestQueue in build_pipeline; the
stub class is kept for Phase 2.1 backward-compat tests.
- enqueue(): adds new events, updates in place by id, detects
resolutions (expires past, or title contains cleared/reopened/
ended/resolved/back online/recovered/lifted)
- tick(now): rolls expired actives into since_last
- render_digest(now): produces a Digest with mesh_compact (<=200
chars) and full multi-line forms; clears since_last after
- Toggle ordering and labels match the v0.3 design
- Phase 2.3b will add real scheduling on top of this
