test(session): bind tool-result parity to retained V3 replay

The parity assertion previously compared a fixed V3 log with whichever generation the ordinary scenario selected. A legitimate V4 re-recording could change call ids and make the assertion compare different model inputs.

Run an additional replay-only variant through the existing headless runner with both its replay file and migration expectation bound to the retained V3 recording. Keep the ordinary scenario on its highest generation, retain call ids in the comparison, and preserve every committed recording.

Validation: the ordinary replay, retained-V3 replay, and old-wrapper negative control pass. Changing only the latest V4 call ids makes the former assertion fail but leaves the corrected retained-V3 replay passing; both fixture and source bytes are restored after this control.

snapshots/session/headless.snapshot.ts

@@ -156,7 +156,7 @@ function records(log: string): JsonObject[] {
     .map(line => JSON.parse(line) as JsonObject)
 }
 
-/** Compare migrated expectations with native writer fields before any format-normalizing comparison. */
+/** Compare deterministic recorded tool output with raw writer fields; only message ids are volatile. */
 function verifyToolResultWriterParity(fixture: string, actual: string): void {
   expect(sessionHeaderVersion(fixture, 'retained tool-result input')).toBe(3)
   expect(sessionHeaderVersion(actual, 'raw tool-result writer')).toBe(SESSION_FORMAT_VERSION)
@@ -211,8 +211,8 @@ async function persistedSessions(cwd: string): Promise<SessionLog[]> {
   })
 }
 
-async function fixtureSessions(scenario: HeadlessScenario): Promise<string[]> {
-  const files = sessionFixtureNames(await readdir(scenario.dir))
+async function fixtureSessions(scenario: HeadlessScenario, selectedFiles?: readonly string[]): Promise<string[]> {
+  const files = selectedFiles ?? sessionFixtureNames(await readdir(scenario.dir))
   return Promise.all(files.map(async (file) => {
     const content = await readFile(join(scenario.dir, file), 'utf8')
     assertSessionFixtureVersion(file, content)
@@ -1063,14 +1063,23 @@ describe('headless recorded-session snapshots', () => {
       .toThrow('native tool-result messages match migration output')
   })
 
-  for (const scenario of scenarios) {
+  const runs = scenarios.flatMap((scenario): { scenario: HeadlessScenario; retainedToolInput?: string }[] => [
+    { scenario },
+    // This comparison retains its own V3 input even after the ordinary scenario records a newer generation.
+    ...mode === 'replay' && scenario.name === 'tool-call-turn'
+      ? [{ scenario, retainedToolInput: 'session.v3.jsonl' }] : [],
+  ])
+  for (const { scenario, retainedToolInput } of runs) {
     const skipped = scenario.manifest.platform === 'posix' && process.platform === 'win32'
       || scenario.manifest.platform === 'pwsh' && !hasPwsh
       || mode === 'record' && scenario.manifest.recording === 'authored'
       || mode === 'record' && scenario.manifest.sessionFormat !== undefined
     const scenarioTest = skipped ? it.skip : mode === 'replay' ? it.concurrent : it
-    scenarioTest(`${mode}s ${scenario.name} through dsh --profile headless`, async () => {
-      let fixtures = await fixtureSessions(scenario)
+    const inputLabel = retainedToolInput === undefined ? '' : ' from retained V3 input'
+    scenarioTest(`${mode}s ${scenario.name}${inputLabel} through dsh --profile headless`, async () => {
+      let fixtureFiles = retainedToolInput === undefined
+        ? sessionFixtureNames(await readdir(scenario.dir)) : [retainedToolInput]
+      let fixtures = await fixtureSessions(scenario, fixtureFiles)
       const primaryFixture = fixtures[0]
       if (primaryFixture === undefined) throw new Error(`${scenario.name}: missing primary session fixture`)
       const task = taskFromSession(primaryFixture) ?? scenario.manifest.input?.task
@@ -1085,7 +1094,6 @@ describe('headless recorded-session snapshots', () => {
       const composition = ownerOf(scenario)
       const baseComposition = compositionOwners.get('default')
       if (baseComposition === undefined) throw new Error('headless corpus has no default composition')
-      let fixtureFiles = sessionFixtureNames(await readdir(scenario.dir))
       const replaying = mode !== 'record'
       const compositionPatch = join(composition.dir, replaying ? 'cordis.snapshot.yml' : 'cordis.yml')
       const patchSources = [
@@ -1167,9 +1175,8 @@ describe('headless recorded-session snapshots', () => {
           },
           inspect: async (cwd) => {
             actualLogs = await persistedSessions(cwd)
-            if (replaying && scenario.name === 'tool-call-turn') {
-              const retainedV3 = await readFile(join(scenario.dir, 'session.v3.jsonl'), 'utf8')
-              verifyToolResultWriterParity(retainedV3, actualLogs[0]!.content)
+            if (retainedToolInput !== undefined) {
+              verifyToolResultWriterParity(primaryFixture, actualLogs[0]!.content)
             }
             if (mcpDemo !== undefined) {
               const log = actualLogs[0]!.content