test(snapshot): share identical header sidecars

.agents/notes/implemented/testing/2026-06-19-acp-snapshot-tests.md

@@ -53,7 +53,7 @@ Replay uses a `cordis.snapshot.yml` overlay that replaces the real adapter with
 A snapshot run asserts **two** normalized surfaces, because the harness's external surfaces are distinct:
 
 1. The **stdout transcript** — the framed ACP JSON-RPC responses and committed-message updates an automation client receives. It catches regressions in the transport contract and is compared against a committed `stdout.expected.jsonl`.
-2. The **re-persisted session JSONL**, normalized and compared with `session.jsonl`. The same fixture is both replay source and expected log. Prompt text is scrubbed; one scenario per header class pins readable prompt and tool content as described in the [header-pinning Agent Note](../../archived/testing/2026-07-06-pin-request-header-content-in-one-scenario.md). Override scenarios derive model behavior solely from their sidecar.
+2. The **re-persisted session JSONL**, normalized and compared with `session.jsonl`. The same fixture is both replay source and expected log. Prompt and tool bulk are scrubbed; one scenario per header class pins the remaining header sequence. The pin owns readable prompt and tool-schema sidecars by default, or names another pin as either source when the complete sequence is identical, so each distinct sidecar version is committed once. Fixture guards reject duplicate sidecar content, and record/refresh rejects shared claimants that generate different bytes. The original header-pinning rationale is preserved in the [header-pinning Agent Note](../../archived/testing/2026-07-06-pin-request-header-content-in-one-scenario.md). Override scenarios derive model behavior solely from their sidecar.
 
 The surfaces are complementary: stdout covers the minimal automation wire, while JSONL covers loop, tool, and boundary structure that the wire intentionally omits.
 
@@ -76,9 +76,10 @@ Tool determinism comes from a generated cwd, scrubbed environment, fresh non-log
 - **A hand-authored `llm.json` of model chunks** — the earlier draft; reusing the real session log makes the fixture a genuine product of the system rather than a hand-built mock, and doubles it as a behavioral expected output.
 - **A byte-level HTTP-record library (Polly/nock/MSW)** — rejected: adapter-specific, awkward with streaming SSE, and lower-level than the thing under test.
 - **Synthesizing throw/cancel entries from `turn/end {kind:'error'|'aborted'}`** — rejected: it couples `llm-replay` to loop-internal turn-closing semantics, and the `turn/end` reason is lossy (it cannot distinguish a thrown 401 from a finish-error); the explicit `replay.override.json` sidecar is the cleaner seam.
+- **Copying both request-header sidecars beside every class pin** — rejected: prompt and tool-schema composition vary independently, so a change to one shared component would churn byte-identical files across unrelated class pins. Explicit per-component sources retain one structural pin per class without duplicating content.
 
 ## Consequences
 
-The tier adds reviewed per-scenario input, session, stdout, optional override, and optional workspace fixtures. Workspace seeds are copied into the generated cwd for both record and replay. In return the tier provides deterministic keyless coverage through the real Loader and tool composition. Most retained scenarios exercise the assembled backend rather than ACP; the [automation-only ACP decision](../simplification/2026-07-23-acp-automation-only-protocol.md#snapshot-boundary) keeps that corpus here and defers any move to a transport-neutral headless suite as an independent testing change (the suite-level FIXME marks it).
+The tier adds reviewed per-scenario input, session, stdout, optional override, and optional workspace fixtures, plus one file for each distinct pinned prompt and tool-schema sequence. Workspace seeds are copied into the generated cwd for both record and replay. In return the tier provides deterministic keyless coverage through the real Loader and tool composition. Most retained scenarios exercise the assembled backend rather than ACP; the [automation-only ACP decision](../simplification/2026-07-23-acp-automation-only-protocol.md#snapshot-boundary) keeps that corpus here and defers any move to a transport-neutral headless suite as an independent testing change (the suite-level FIXME marks it).
 
 This Agent Note relates to but does not supersede the [proposed determinism Agent Note](../../proposed/testing/2026-06-11-deterministic-and-stress-testing.md): that proposal's "universal replay fixture" re-derives session *message history* after every test (an internal-consistency invariant), whereas these snapshots pin assembled behavior plus the external automation output. They are complementary until the backend corpus moves off ACP.