fix(ci): let the complexity delta account for itself — the receipt could not see it

[wshallwshall]

The liveness control shipped in #25 had this
workflow's own failure mode inside it.

The complexity receipt proved liveness with ruff --show-files, which counts the files ruff would
enumerate — true whether or not the delta step ran. So a delta that bailed on an unresolvable merge
base emitted a ::notice, exited 0, and left a green measured receipt behind. On a PR the delta is
the point of that job, and it could go quiet while the gate reported success.

The fix

The delta records its own outcome to c901-delta.env — written last on the success path, so its
absence proves the step died, and written explicitly on the bail path, so a skip is distinguishable
from a crash. The receipt rules on it:

Situation	Verdict
PR + delta ran	measured — evidence names the merge base and base/head finding counts
PR + marker missing	failed — "did not complete"
PR + delta bailed	failed — carries the recorded reason
cron / dispatch	measured — "the PR delta does not apply on <event>"

That last row is deliberate: on a non-PR event the delta legitimately doesn't run and the whole-repo
triage is the measurement. Failing there would fire on good news, and a check that fires on good news
gets muted — worse than not having it.

Two bugs found by executing the receipt shell, not reading it

1. The reason was silently discarded. The receipt sources the marker, and the reason was written
   unquoted — bash parsed a value containing spaces as a command and left the variable unset. The
   failure reported "no reason recorded", throwing away the only useful diagnostic on the exact path
   that exists to provide one.
2. My own new test was vacuous. It asserted the marker appeared ≥2 times in the step, which the
   bail path satisfies alone (it writes two lines) — so deleting the success-path marker passed.
   Caught by mutating the workflow and watching the assertion fail to go red. It now partitions the
   step at the c901_delta.py call and requires a marker on both sides.

Verification

All four receipt paths executed against the shell extracted from the YAML; all four negative probes on
the new assertions go red. 55 quality tests pass · ruff check clean · zizmor real exit 0 · YAML valid.

Advisory-only as before: no write scope on any job, no new required check.