fix(harness): assert redacted tool path per host separator

The experiment runner test failed only on windows-latest, expecting
`<trial-root>/README.md` while the run emitted `<trial-root>\README.md`.

The fault was in the test, not the runner. Its fake executor builds
`input.path` with native `join(context.worktree, "README.md")`, and
`redactEvidenceValue` deliberately rewrites just the trial-root prefix and
leaves separators alone. Normalizing separators inside the runner would be
wrong: tool payloads are opaque host data, so a blanket backslash rewrite
would corrupt code content, regex literals, and Windows command lines.

Replace the stringified-blob `toContain` with a structural assertion on the
`tool-call-started` event's `input.path`, normalizing separators at that
boundary. This matches the existing precedent in compare.test.ts and asserts
on parsed event shape rather than on text that happens to contain it.

Also close a latent gap in the neighbouring leak guard: `JSON.stringify`
escapes backslashes, so `not.toContain(root)` could never match a leaked
native Windows root and passed vacuously there. It now also checks the
JSON-escaped form, which is equivalent to the raw check on POSIX.

Validated with the full harness package suite (172 tests, 20 files) and
`tsc --noEmit`. Since a local macOS pass is not Windows evidence, the
runner's redaction logic was also replayed against `path.win32` drive and
UNC roots plus posix; the win32 form reproduces the exact CI symptom and
normalizes correctly. The windows-latest job remains the authoritative
receipt.

Co-authored-by: QoderAI (Qwen 3.8 Max) <qoder_ai@qoder.com>

Author: phodal

packages/harness/test/experiment-runner.test.ts

@@ -174,8 +174,15 @@ describe("checkpoint experiment runner", () => {
       expect.objectContaining({ experimentId: "exp_parallel_test", laneId: "default", runId: "exp_parallel_test:default:1" }),
       expect.objectContaining({ experimentId: "exp_parallel_test", laneId: "minimal", runId: "exp_parallel_test:minimal:1" }),
     ]));
-    expect(JSON.stringify(events)).not.toContain(root);
-    expect(JSON.stringify(events)).toContain("<trial-root>/README.md");
+    const serializedEvents = JSON.stringify(events);
+    expect(serializedEvents).not.toContain(root);
+    expect(serializedEvents).not.toContain(root.replaceAll("\\", "\\\\"));
+    const callEvent = events.find((event) => event.type === "lane-event"
+      && event.event.type === "tool-call-started");
+    // Redaction rewrites the trial root only; tool inputs keep host-native separators.
+    expect(callEvent?.type === "lane-event" && callEvent.event.type === "tool-call-started"
+      ? (callEvent.event.input as { path: string }).path.replaceAll("\\", "/")
+      : null).toBe("<trial-root>/README.md");
     const resultEvent = events.find((event) => event.type === "lane-event"
       && event.event.type === "tool-call-result");
     expect(resultEvent?.type === "lane-event" && resultEvent.event.type === "tool-call-result"