fix(acp-snapshot): make path-separator tests platform-neutral

Three tests in the shared acp-snapshot package hardcoded POSIX path
separators in their assertions, so they failed on Windows where
node:path.join produces backslash paths:

- childFixturePaths (suite.spec.ts): expected literal '/snap/s/session.1.jsonl'
  but join returns '\snap\s\...' on Windows; use join() for the expected value.
- harness.spec.ts (env-forwarding test): substring-matched a JSON-encoded
  path against raw stdout text, where backslash escaping makes the compare
  byte-fragile; parse the env-probe chunk and compare the structured value.
- harness.spec.ts (harvested-cwd test): substring-matched the raw cwd
  against JSONL text where the cwd is JSON-escaped; parse the session line
  and compare the cwd field.

These were master's latent bugs (the package's tests never ran on Windows
until the Windows CI lane observed them). Verified green on Windows via
scripts/caohuanqi-private/run-ci.py --windows.

packages/support/acp-snapshot/tests/harness.spec.ts

@@ -72,7 +72,11 @@ describe('runScenario', () => {
     expect(result.sessionLogs[0]?.createdAt).toBe(42)
     expect(result.sessionLogs[0]?.content).toContain('turn/start')
     // The harvested log embeds the run's REAL temp cwd (template-substituted).
-    expect(result.sessionLogs[0]?.content).toContain(result.cwd)
+    // The cwd is JSON-encoded in the log line, so compare the parsed field
+    // rather than substring-matching a raw path (which breaks when the path
+    // separator is escaped inside JSON text on Windows).
+    const sessionLine = result.sessionLogs[0]?.content.split('\n').find(l => l.includes('"type":"session"')) ?? '{}'
+    expect((JSON.parse(sessionLine) as { cwd?: string }).cwd).toBe(result.cwd)
   })
 
   it('forwards override/child fixture paths into the child env and captures stderr', { timeout: 20_000 }, async () => {
@@ -93,7 +97,17 @@ describe('runScenario', () => {
     expect(result.stderr).toContain('fake bin booted')
     expect(result.rawStdout).toContain('replay.override.json')
     // Child paths ride one env var, joined with the platform delimiter.
-    expect(result.rawStdout).toContain(JSON.stringify(childFiles.join(delimiter)).slice(1, -1))
+    // Parse the fake bin's env-probe chunk rather than substring-matching a
+    // JSON-encoded path (the escaping breaks raw-substring compares on Windows).
+    const envChunk = result.rawStdout.split('\n')
+      .map(l => l.trim())
+      .filter(l => l.length > 0)
+      .map(l => JSON.parse(l) as { params?: { update?: { content?: { text?: string } } } })
+      .find(f => f.params?.update?.content?.text?.startsWith('env:'))
+    const env = JSON.parse((envChunk?.params?.update?.content?.text ?? 'env:{}').slice('env:'.length)) as {
+      childFiles: string | null
+    }
+    expect(env.childFiles).toBe(childFiles.join(delimiter))
   })
 
   it('seeds the workspace dir into the temp cwd before the run', { timeout: 20_000 }, async () => {

packages/support/acp-snapshot/tests/suite.spec.ts

@@ -181,7 +181,7 @@ describe('defineAcpSnapshotSuite: registration contract', () => {
 
 describe('childFixturePaths', () => {
   it('yields one sibling path per child, 1-based', () => {
-    expect(childFixturePaths('/snap/s', 2)).toEqual(['/snap/s/session.1.jsonl', '/snap/s/session.2.jsonl'])
+    expect(childFixturePaths('/snap/s', 2)).toEqual([join('/snap/s', 'session.1.jsonl'), join('/snap/s', 'session.2.jsonl')])
   })
 
   it('yields nothing for a single-session scenario', () => {