A threshold of 3700 seconds against an expected 3779 accepted exactly the state
the pass sits in for minutes: it writes in 120 s chunks, so 31 chunks is 3720 s —
one short — and "basically done" sails through a loose comparison. My own monitor
fired a premature COMPLETE on that very number, which is how I noticed: the check
and the bug had the same blind spot because I wrote both.

It now reads the expected duration from the master itself and allows two seconds,
so the check cannot drift from the thing it checks.

The fallback message says what a short global file would actually cost, which is
the part worth naming: it does not FAIL the comparison, because compare_global
takes min(len(a), len(b)). It compares fewer seconds per track and reports thinner
evidence under identical column headings. Silent degradation in a summary nobody
watches being written is worse than an error.

Second attempt at this commit. The first edit silently no-op'd — a scripted
replacement whose search string carried escaped line-continuations that did not
match the file, while the script printed "gate tightened" unconditionally, and I
reported success on the strength of that print. Caught by grepping the file. This
version asserts the anchor exists before replacing and asserts the content changed
after, then proves it by grep rather than by message. That is the third instance
tonight of the same failure shape — a check whose failure mode is indistinguishable
from success — so the assertions are the actual fix, not the gate.

Held until now deliberately: finish_cosmic.sh was mid-execution, and bash reads
scripts incrementally, so editing a running script can corrupt the run.