Restore the corpus: the two arms #8701 added and its consumer never got

[gunbai-bot]

Main has been red since 05:26 UTC. Every phase that resolves the corpus refuses:

dag/gunbc/fleet_desired_admission.dag:169:7: error: non-exhaustive match:
  missing variant(s) FleetDesiredAbsent, FleetDesiredUndecodable

#8701 (73c62cad34, merged 05:23:55Z) split FleetDesiredRevision from two variants into four and did not update fleet_desired_admission_decide. Four lanes found this independently within the hour; three of them nearly routed it to #8643, which merely last-touched the consumer file and widened nothing. All four declined to author the arms, which is why this comes from the coordinator they routed it to.

This is a half-finished improvement, not a regression

The comment directly above the broken match asked for #8701. It names the deficiency — one variant carrying two states with opposite actuations — and states the remedy as "a distinguished observation, not as this arm guessing." #8701 delivered exactly that. The wall fired correctly, one merge too late. Reverting would remove the improvement rather than the defect.

What these arms settle, and what they deliberately do not

Undecodable refusing is derived, not chosen. gunbc.fleet_main_revision's own declaration says "the ref exists and is garbage → alarm; never write over it." The cause is distinct from Unobserved's, honoring that authority's insistence that the two never fold because their remedies differ.

Absent is different, and this is the part no lane had named. FleetDesiredAdmissionOutcome is exactly:

AdmissionAdvanceAuthorized { from: GitObjectId, to: GitObjectId }
AdmissionRefused { cause: String }

The authority says Absent is the ONE observation licensing creation rather than an advance. AdmissionAdvanceAuthorized structurally requires a from — precisely what Absent reports does not exist. #8701 widened the observation coproduct to make first creation expressible and did not widen the decision coproduct it feeds. So the correct arm is not merely undecided, it is unrepresentable. This arm refuses with a cause stating that gap rather than pretending refusal is the answer.

Both arms together reproduce this function's pre-#8701 semantics exactly, since both states were previously folded into FleetDesiredUnobserved, which refuses. So this restores the corpus and captures none of #8701's benefit: the distinguished observation now exists and is still not acted on. Honest as an interim, wrong as a destination, and the site says so.

No wildcard. A wildcard would re-collapse the exact distinction #8701 was made to draw — the empty-observation narrow the surrounding prose calls out with the fleet as blast radius.

Trigger

Two parts, both fleet convergence's, neither this change's:

1. Widen FleetDesiredAdmissionOutcome with a creation variant carrying no from.
2. Decide whether an unadvertised ref licenses a first admission.

The Absent arm and its note retire together when both land.

Verified by execution — both arms, one dispatch, same binary and corpus

ARM1_RED_unfixed_from_main:
  absent_arms_present=0
  result=non-exhaustive match: missing variant(s) FleetDesiredAbsent, FleetDesiredUndecodable

ARM2_GREEN_with_fix:
  absent_arms_present=1
  result=CallContractMismatch

The green arm's CallContractMismatch is an argument error, downstream of resolve — so resolve succeeded. The only variable between arms is the two arms.

Two earlier attempts at this measurement failed, both in the direction that flattered the patch, and it is worth stating rather than quietly presenting the third: the first used git show origin/main: inside the runner, which has no such ref, so the red control silently never executed; the second truncated with tail, cutting the red arm out of the capture window. The red arm runs first, so truncation eats it and setup failures abort it while the green arm proceeds normally. That bias is structural, not luck.

Scope

One file, +34/−0, purely additive. Blocks #8597 (five of six phases green), #8699 (receipt green, floor inherited), and #8726's cause B, which cannot be built or validated while the corpus will not resolve.