feat(tide-table): sc_generation_lens — which cut is actually on the account

PLN opened the day hearing it: 'our private cuts of OPAL are badly cut
begin/end unaligned'. The board had one theory (a 24s truncation) and it was
wrong. This lens found the real shape: the ENTIRE Aug-12 first cut is still
on SoundCloud — all 15 tracks — living beside the 6 correct v4 ones under a
litter of ad-hoc permalinks (/opal26-wap, /12-mafia-12, /ceci-nest-pas-une-
bombe), and eight of the stale ones are the ones wearing artwork. So the
wrong cut is the one that looks finished. Of course he heard it.

Duration is the discriminator, and it is far sharper than assumed: measured
on this account, SoundCloud's full_duration reproduces the source to the
millisecond (477.600 uploaded, 477.600 reported). Tolerance is 50ms, not the
seconds you would allow for a transcode — a loose tolerance here buys no
safety, it only invents ambiguity between cuts that differ plainly.

Three things I got wrong first, each left in as a guard:
  - filenames are not authoritative. 'Perfect <3' uploads to /perfect-3, so a
    filename-derived slug reported two live tracks as NOT UPLOADED. Titles
    come from the plan now, which is what the uploader actually used.
  - streaming and club masters share every duration, so offering both as
    rival candidates marked all 14 AMBIGUOUS. Identical generations collapse.
  - a duration collision is not provenance. /au-revoir-lord-toyota (2020) and
    /saria-demo (2025) both matched OPAL to 50ms. Anything uploaded before
    the oldest render on disk cannot be a cut of it, and a coincidence must
    never reach a delete manifest — so the date floor is a hard gate and the
    discards are printed, not silently dropped.

The second pass is the one that earned its keep. Roll call only looks where
it expects a track to be, so it can never see a stale cut hiding under a
different slug. Strays scans the whole account. Absence needs proof.

    fifteen ghosts of a set played once,
    each wearing the cover art of the real thing --
    the lens does not listen, it only counts seconds,
    and seconds were enough.