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.

docs/proposals/_BLOCKED-BY-CONSTRAINT.md

@@ -215,4 +215,4 @@ Linus 最后一句：**"先让数据结构对，其它 6 步会一起变简单"
 - No production mocks / fallbacks / silent catches — 仅 dev `tests/eval/` 例外。
 - No runtime logging / telemetry beyond existing electron-log usage.
 - No dual-write migrations without backup path。
-- `tests/eval/` 必须在 `electron-builder.yml` 的 files 过滤中排除，保证不进生产 bundle。
+- `electron-builder.yml` 采用 whitelist `files:` 模式（`out/**` + `package.json` + 指定 assets），`tests/eval/` 本就不在白名单内，无需新增排除规则。