AUDIT-WP-0018-T03: report bounded-executor runs that started without a receipt

A sender may declare closing_classes; nk-bounded-executor declares
execution.run.started closed by execution.attribution.receipt within
7200s. GET /v1/stream-findings reports opening_without_closing past the
bound, counted from the receiver's accepted_at plus the heartbeat grace
allowance; a per-run data.window.not_after tightens but never extends.
Identifiers and timing only. The overlay may add or shorten, never
remove, rebind or lengthen. Both backends: 403 passed (1 unrelated
live-catalog env failure).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 482098@bnt-lap001
Assistant-Session: 9643c95a-a398-4f4a-b56a-6119be4e845e

docs/stream-completeness.md

@@ -180,3 +180,48 @@ schema is identical). The result, stated at no more than it is:
   onto audit-core's evaluation by itself: a source needs `heartbeat_classes` on
   its registration too, and the two can drift apart. That is fixable on
   either side and has not been fixed on either.
+
+## Opening events that never close (AUDIT-WP-0018-T03)
+
+Some streams come in pairs. The bounded executor emits
+`execution.run.started` once it has consumed its single approval, and
+`execution.attribution.receipt` when the run ends. A crash in between leaves
+only the start in custody, and neither heartbeats nor rate counts can see that.
+
+A sender's registration may therefore declare `closing_classes`:
+
+```json
+"closing_classes": {
+  "execution.run.started": {
+    "closed_by": "execution.attribution.receipt",
+    "within_seconds": 7200
+  }
+}
+```
+
+`GET /v1/stream-findings` then carries an `opening_without_closing` finding
+for every opening event that has no closing event with the same
+`correlation_id` from the same source by its deadline.
+
+- **Clock.** The deadline counts from the receiver's own `accepted_at`, never
+  from a source-supplied time.
+- **Bound.** The deadline is the declared `within_seconds`. If the opening event
+  carries `data.window.not_after`, the earlier of the two applies (`bound` names
+  which one). A per-run window can tighten the declaration but never extend it.
+- **Grace.** The same 1.5× allowance as heartbeats is added
+  (`within_seconds × 0.5`), so a receipt accepted moments late does not flap
+  the finding. For the executor that means a finding at 3 h at the latest.
+- **Lookback.** Opening events accepted in the last 30 days, the operational
+  custody window, are examined.
+- **No correlation id.** An opening event without a `correlation_id` can never
+  be closed and is always reported.
+- **Disclosure.** A finding carries identifiers and timing only: event id,
+  correlation id, accepted time, deadline. No payload is disclosed.
+- **Overlay.** The scope overlay may add a pair or shorten its bound. It may
+  never remove a pair, rebind its closing class, or lengthen the bound.
+
+The finding's `means` says what it does not prove. The run may have partly
+executed, failed before emitting, or had its receipt lost or suppressed.
+Audit-core does not infer which, and does not infer what the run did. It is a
+finding for the verifier and the founder. As with heartbeats, a compromised
+emitter that suppresses its start and its receipt together is not covered.