docs(backlog): one witness per e2e attempt, and twenty seconds

[G1NG4R]

Backlog only — no code. The measurements around #458, including one swap that
got away and the rule that follows from it.

What it carries

• The f0e5998 swap is unobserved, and the cause was my scheduling. The
  witness was armed while e2e attempt 1 was running; that attempt failed,
  so deploy was gated off and its 2700 s rail burned down on a window where no
  swap could occur. It expired at 03:46:06. The rerun's deploy began at
  03:46:26 — twenty seconds later. A second witness started at 03:51 and
  measured what was already over.
• The rule: one witness per e2e attempt, re-armed until the target sha is
  actually serving. It worked immediately on the next swap — armed 04:13:34,
  twelve seconds after e2e began, and the swap came in clean: 934 × 200 per
  path, zero non-200, offset +15.8 s.
• v0.47.3 is the night's first release, because fix is a releasing type
  where the six preceding chore and docs merges correctly produced none.
• p95 read 241.2 ms, measured five minutes after the restart. That is the
  docs(backlog): the watcher waited for itself, and p95 was stale #452 shape, and the entry says so without claiming a second reading: both
  of my reads returned the same measuredAt, so this is explained by precedent
  and arithmetic, not disproven by a fresh measurement.
• Two tooling errors of my own, same family: a kill that returned 0 while
  the witness kept sampling (I had killed the wrapper, not the witness — a
  return code is not proof), and printf '--- …', whose format string begins
  with a dash and is read as an option. The second is why the coverage figure
  for test: the witness tail is measured from the twin, and closes before #304's window #453's sixth data point is derived rather than read.

Tally: 6 drops in roughly 41 430 requests, thirteen swap runs in a row without
one. A fifth no-deploy window was discarded rather than counted — it had no
deploy ahead of it and was stopped so the next witness could not run beside it.

make check green.