feat(web): add turn-scoped cancellation

[testikun]

Adds a Stop control for the current Pi agent execution without stopping the Web Host. Requests bind to the Session, command, and execution epoch, so stale tabs cannot cancel a later execution.

Closes #342.

The runtime invokes native AgentSession.abort() and waits for agent_settled. Individual assistant messages, including toolUse, do not establish execution completion. Cancellation needs an aborted assistant result; missing terminal evidence remains uncertain. The browser restores the control from snapshots and SSE and does not optimistically claim completion. Queued follow-ups remain under Pi ownership and may continue within the same execution; this does not introduce per-input cancellation or a second queue.

Validation: bun run check passed; final integrated focused Web tests passed 75/75. The repair's standalone Node 22 full suite passed 1405 tests with one skip, plus Vitest 30/30. CI is running against the updated head. Earlier live-provider evidence in the issue predates this repair and is not a new runtime acceptance result.

[testikun]

CI status note: Node 22/24 completed this PR's repository checks and failed only at the final fresh-install Smoke-test packed standalone Web CLI step, where the child exits before the test can read its URL. The same failure now occurs on unrelated PRs #385, #386, and #387.

I reproduced the packed install independently: npm now admits Pi 0.85.0 through OpenPI's published peerDependencies: "*"; the CLI then exits with Cannot find module '@earendil-works/pi-server'. The checkout/lock-based bun run check and full test suite remain green. I recorded the shared packaging regression and suggested peer-range follow-up in #328: #328 (comment)

No dependency workaround is mixed into this feature PR.

[tt-a1i]

方向对，有一处应修；请用这份替换 #358

turn 范围（sessionId+commandId+epoch）、等 turn_settled(outcome=cancelled)、不乐观结算，这些都符合合同。#358 从 abort() 返回值推断取消，请关掉那份。

应修：cancelActiveTurn 里 Promise.race([settlement, abortFailure])。如果 abort() resolve 了，但又没有 assistant message_end，HTTP 会一直挂着（没有超时）。请给 settlement 加界，或把「abort 成功但未结算」变成明确的非终态/错误，而不是挂起。

CI 红是共享的打包 openpi web 冒烟，不是这份 diff 独有的。

[tt-a1i]

需要改：abort() 成功但没有 message_end 时 HTTP 会一直挂

上一评写成 COMMENT 偏软。cancelActiveTurn 里：

const abortFailure = new Promise<never>((_, reject) => {
  void abortOperation.catch(reject);
});
const terminal = await Promise.race([settlement, abortFailure]);

abort() resolve、又没有 assistant message_end 时，settlement 永不 settle，abortFailure 也不 reject，HTTP 无限等。这是取消合同上的无界挂起，合入前要加超时或明确的非终态/错误。

方向仍对：sessionId+commandId+epoch、等 turn_settled、替换 #358。CI 红仍是共享打包冒烟。

[testikun]

已按 review 修复并推送到 c4305ad：取消请求现在同时等待 settlement、abort 失败和 10 秒有界超时；abort 成功但没有 message_end/turn_settled 时会返回明确的 failed 错误，不会让 HTTP 请求无限挂起。原有 Pi runtime 测试 22/22 通过，diff 检查通过。