feat: add non-provisioning sandbox stop lifecycle

[codyss]

Summary

Add a non-provisioning SessionContext.stopSandbox() lifecycle API for durable session boundaries.

The API stops the current session's owned compute without calling getSandbox(), creating a provider resource, resuming a stopped resource, running onSession, or running base-runtime initialization. Persisted session metadata remains available for the next callback.

Implementation

• Add SessionContext.stopSandbox() with idempotent and ownership-aware behavior.
• Add the optional backend stopExisting primitive and public input/result types.
• Coalesce concurrent stop requests within one access and tolerate replayed requests.
• Add the Vercel implementation using Sandbox.get({ resume: false }) and stop only running or pending resources.
• Preserve the existing explicit live-handle getSandbox().stop() behavior.
• Document the lifecycle API and add unit and integration coverage for unused, persisted, concurrent, shared, missing, already-stopped, and provider-failure cases.

Verification

• pnpm exec oxfmt --check .
• pnpm --filter eve typecheck
• pnpm --filter eve build:js
• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/execution/sandbox/ensure.test.ts src/execution/sandbox/bindings/vercel.test.ts
• pnpm --filter eve exec vitest run --config vitest.integration.config.ts src/context/accessors.integration.test.ts

The full unit suite currently has four environment-sensitive failures unrelated to this change: three existing redaction assertions include the checkout's /private path, and one existing production-server test cannot bind loopback in the restricted test environment. The focused lifecycle and Vercel suites pass.

[vercel]

@codyss is attempting to deploy a commit to the Vercel Team on Vercel.

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[codyss]

Ready for review after adding the DCO sign-off.

This API is needed by Dogsled to stop Vercel Sandbox compute at durable session boundaries without provisioning or resuming a resource.