Legacy/migrated items with no watchedAt were siloed into a literal
"1970-01" month event that self-perpetuated (read back as 0 -> re-bucketed
-> re-published with 0). Derive a stable watch time from the pointer
d-tag's embedded ms-timestamp and backfill it so they land in a real month
and the loop stops. Bucketing extracted to watchHistoryBucketing.js to keep
watchHistory.js under its size cap.
Also fixes three stale tests in watch-history.test.mjs (partial-acceptance,
actor alignment, session flag) so the file is un-quarantined.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
