The previous commit fixed three bugs in the tool whose entire job is catching
failures that make no noise. That makes its own failure mode the worst one
available: saying OK about a track that does not work, or BROKEN about a track
that does. It was doing BOTH, corpus-wide — and it stayed hidden because the 13
setlist tracks, the only ones anyone ever pointed it at, happened to dodge all
three.

Seven tests, all structural — no ghc, no rig, 0.06s:

  1. a column-0 `$` is a CONTINUATION (PLN's dominant corpus style), plus a
     guard that indenting does not FLATTEN relative structure below it, plus
     one for the ordinary inline style — which gets a test precisely because
     dodging the bug is what let the bug live.
  2. a non-zero exit with no SILENT line is a CRASH, and its stderr survives
     into the detail, because stderr is the only thing that says why.
  3. the exit code answers "is anything wrong", never "is anything silent".

MUTATION-CHECKED, because green tests on their own prove nothing. Each fix was
individually reverted and the suite re-run:

  remove the continuation indent  -> test_dollar_at_column_zero_is_indented FAILS
  key the return to silence only  -> test_exit_code_is_not_keyed_to_silence_alone FAILS
  both restored                   -> 7 passed

Test 3 is the one that matters. When crashes were split out of the silence
count, the return value was briefly left keyed to `silent_only`, so a corpus
where every single track failed to compile would have printed "OK — every
declared orbit emits events" and exited 0. That is a false green from the gate,
and a false green here is indistinguishable from a working set right up until
the downbeat.

Full suite: 229 passed.