feat: implement durable event activities

[miyaontherelay]

Implements the authored Surface and SDK portion of docs/EVENT-AWAIT.md.

This draft adds Ctx.on() with bounded Activity.next()/close(), validates required idle and deadline, lowers subscription open/next/close messages, strictly decodes wake results, and closes activities with the run lifecycle. It intentionally does not claim the kernel timer, durable stream, Cloud router, ingress replay, dedupe, overflow, or recovery work; those slices are continuing on this branch.

Validation completed:

• packages/surface: typecheck, build, and focused activity test
• packages/sdk: typecheck and 34 focused activity/authored-flow tests
• git diff --check

Detailed commands and results: docs/evidence/event-await-implementation/surface-sdk.md.

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[github-actions]

Review swarm: maintainability

No fresh transcript was produced for run 6f3da1db-bca8-4122-94b2-490dd7bee241 (MISSING).

[github-actions]

Review swarm: history

No fresh transcript was produced for run 6f3da1db-bca8-4122-94b2-490dd7bee241 (MISSING).

[github-actions]

Review swarm: structure

No fresh transcript was produced for run 6f3da1db-bca8-4122-94b2-490dd7bee241 (MISSING).

[github-actions]

Review swarm: FAILED

• maintainability: MISSING
• history: MISSING
• structure: MISSING

Cloud run: 6f3da1db-bca8-4122-94b2-490dd7bee241

[khaliqgant]

Consolidated into #441, which now targets main and includes this branch’s implementation. Closing the redundant base PR; #441 remains draft while independent review and Cloud integration are outstanding. No branch deleted.