B5 harness iteration: self-test + assertion-judge cleanup + dead branch fix + _BLOCKED timeline correction

- harness.test.ts: add in-memory self-test layer (pass + fail + purity), guard
  fixtures dir with existsSync so absent dir yields empty list without
  crashing the runner. Empty-fixture pnpm test:eval now carries real signal.
- replay-client.ts: remove dead if (!turn) branch; cursor bounds-check above
  already guarantees the element. Use non-null assertion with a comment.
- assertion-judge.ts: drop unnecessary (turn as AssistantTurn | { content:
  unknown[] }) cast. turn.role === assertion.role already narrows to
  UserTurn | AssistantTurn and both expose .content.
- _BLOCKED-BY-CONSTRAINT.md: correct electron-builder note. files: is a
  whitelist, so tests/eval is automatically excluded and no new rule is
  required.

tests/eval/replay-client.ts

@@ -90,13 +90,9 @@ export function createReplayStreamFn(fixture: FixtureFile): StreamFn {
         `replay-client: fixture "${fixture.meta.fixtureId}" exhausted after ${assistantTurns.length} assistant turns; agent requested turn ${cursor + 1}`,
       );
     }
-    const turn = assistantTurns[cursor];
+    // Cursor is bounds-checked above, so the element is guaranteed to exist.
+    const turn = assistantTurns[cursor]!;
     cursor += 1;
-    if (!turn) {
-      throw new Error(
-        `replay-client: internal cursor error at position ${cursor - 1}`,
-      );
-    }
     const message = buildAssistantMessage(
       turn,
       model.id,