fix(dates): resume cursor so unconvertible rows can't stall the #118 drain

Review follow-up: rows that exist but can't be faithfully resolved spent
the cap and were re-collected first every tick. Carry a per-library
cursor across ticks (wrapping at end of table), share the cap parser
between the two date drains, and name the conversion tuple.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -446,9 +446,11 @@ partial index `idx_image_exif_utc_backfill`) by re-running
 batch) and writing **only** the new-contract triple — the legacy pair
 stays frozen. Legacy `manual` rows convert via
 `camera_tz::manual_date_resolution` instead of being re-resolved from
-the file. Missing-file rows are paged past without spending the cap;
-EXIF/exiftool-legacy rows wait when exiftool is absent (they'd otherwise
-fall to fs_time). When a converted row's display-zone day differs from
+the file. Missing-file rows are paged past without spending the cap, and
+a per-library resume cursor (carried across ticks by the watcher, wraps
+at end of table) keeps unconvertible rows from stalling the rows behind
+them; EXIF/exiftool-legacy rows wait when exiftool is absent (they'd
+otherwise fall to fs_time). When a converted row's display-zone day differs from
 the day its legacy value rendered on, the `day`-span `precomputed_reels`
 rows covering either month-day (this library + `all`) are deleted and
 unlinked; the nightly pre-gen regenerates. Daily conversation summaries