NK-WP-0056-T15: receipt schema v0.1, validator and executor fixtures; audit sender exercise

- canon/schemas/execution-attribution-receipt_v0.1.schema.json: the bounded-executor profile
  of the PCC v0.2 receipt, both audit-core events (started, receipt), every object closed.
- tools/execution-attribution-receipt: EAR-1..EAR-10 validator, fixtures generated from the
  real executor (completed, reverted, completed-on-retry) with a drift check.
- executor: started carries receipt_version, digests, steps_planned and data.window.not_after
  (audit-core AUDIT-WP-0018-T03 request); receipt carries receipt_version and initiator.iss
  (configured approver_issuer, else unknown).
- bounded-executor/exercise_audit_sender.py: real executor and client against audit-core's
  real ingestion and the Git scope overlay: 202, duplicate 200, conflict 409 as a defect,
  wrong source/tenant/token refused and not stored, a receipt-less start flagged by its own
  window.

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

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 523928@bnt-lap001
Assistant-Session: 6c737afb-9c14-4152-9f02-477214b9c350

tools/execution-attribution-receipt/fixtures/valid/reverted-started.json

@@ -0,0 +1,24 @@
+{
+  "correlation_id": "1c8f5b63-4d2e-4a7f-9ba1-2c3d4e5f6071",
+  "data": {
+    "admission": "BEA-2026-0001",
+    "admission_digest": "sha256:bbaa3afb2885cc1dd8a4b00f48f3f7265aaf05d4c2f03479fb935c44e5bf013b",
+    "approval": "6f1c2b9e-0a4d-4e3b-9c51-7d2f8a6b3e10",
+    "consumed_at": "2026-11-03T09:30:00Z",
+    "executing_workload": "service:nk-bind-executor",
+    "plan_digest": "sha256:df214e02c24e70c6ac799bee873862f0d24d9bd537bf77cfb845a7b880c7565b",
+    "receipt_version": "0.1",
+    "run_id": "1c8f5b63-4d2e-4a7f-9ba1-2c3d4e5f6071",
+    "runtime_principal": "system:serviceaccount:sso:nk-bind-executor",
+    "steps_planned": 4,
+    "window": {
+      "not_after": "2026-11-03T11:00:00Z"
+    }
+  },
+  "id": "run:1c8f5b63-4d2e-4a7f-9ba1-2c3d4e5f6071:started",
+  "occurred_at": "2026-11-03T09:30:00+00:00",
+  "source": "net-kingdom.bounded-executor",
+  "subject": "admission:BEA-2026-0001",
+  "tenant": "platform",
+  "type": "execution.run.started"
+}