#936 added fragmented-row grouping. two follow-ups found in review:
1) BUG: an excluded canonical sibling (a row pinned to the same canonical edition whose tracks
fail the strict fragment match) was emitted with canonical_items but no owned set, so
_build_missing_tracks flagged the ENTIRE tracklist as missing — including tracks the row
already owns (e.g. a 3-track fragment of a 12-track edition reported 12 missing, not 9).
now compute that sibling's own owned slots from its local tracks (shared
_owned_reference_for_tracks helper, same logic as the anchor) so it reports only what it's
actually missing and the count stays internally consistent.
2) PERF: _build_candidate_groups rescanned all albums for every canonical group — O(G*N), which
degrades badly once many editions are pinned (fine today at G=1, latent at scale). invert to
a single O(N) pass that assigns each row to its unique group; identical 'exactly one match'
semantics. also added a Stop/Pause check in _prepare_work_items, which now front-loads the
canonical lookups + matching.
3 new tests (excluded-sibling reports only its missing tracks; ambiguous candidate stays
independent; unambiguous candidate joins) — 17 completeness + 123 repair tests green.
