feat(api): add durable execution receipts

[m-mohamed]

Stacked on #1912.

Summary

• require bounded caller idempotency keys and issue stable execution identifiers
• persist owner-scoped running, paused, and immutable completed execution state
• add typed GET readback with request/result hashes and completion timestamps
• return typed in-progress and conflict responses for safe submission and resume replay
• preserve execution identity across multiple pauses and protect each resume sequence from duplicate execution
• add atomic FumaDB and R2 reservation support

Verification

• bun run typecheck (45 packages, uncached)
• changed-file oxlint with warnings denied
• changed-file oxfmt check
• SDK receipt/blob tests (16)
• execution engine tests (40)
• React shell host tests (10)
• self-host execution HTTP and owner-isolation tests (8)
• workerd/Miniflare Cloudflare host tests with local D1 and R2 (11)