v0.3.0

Compaction boundary and a participant service for other plugins.

• A recall now names where history stops after a compaction: the listing, an out-of-range answer, the picker and the confirmation dialog all say that messages before the newest checkpoint can no longer be recalled.
• The host half publishes dsh-rewind-participants; the file revert is its first participant. A participant that throws or misses its deadline is logged and skipped, never failing the recall.

Verified: npm run build && npm run typecheck && npm test (181/181). Published to npm as @jochenyang/dsh-rewind-plugin@0.3.0.