docs(examples): sandbox-stream-backend uses the real sandbox-SDK SandboxEvent shape (post-#36)

[drewstone]

Follow-up to #36 (closes #35).

The sandbox-stream-backend example previously dodged the load-bearing question by emitting flat { type: 'text_delta', text } — which the default mapper trivially passes through. That was hiding the actual sandbox-SDK event shape (message.part.updated with nested data.part.text) — which is what consumers using the real @tangle-network/sandbox SDK actually hit, and which was the exact gap #36 fixed.

This PR rewrites the example to emit the canonical sandbox-SDK shape verbatim:

yield { type: 'message.part.updated', data: { part: { type: 'text', text: 'hello\n' } } }
yield { type: 'tool_call',  data: { id: 'call_1', name: 'Read', input: { path: '/tmp/notes.md' } } }
yield { type: 'tool_result', data: { id: 'call_1', name: 'Read', output: '(...)' } }

…and demonstrates that createSandboxPromptBackend with no mapEvent shim produces the full canonical RuntimeStreamEvent stream out of the box. Header + inline comments call out:

• the canonical shape variants the default mapper handles,
• that mapEvent is optional (only for product-specific event types outside the documented vocabulary),
• the history of the gap (createSandboxPromptBackend default mapper doesn't unwrap sandbox-SDK SandboxEvent shape (silent text_delta loss) #35) + its fix (fix(backends): mapCommonBackendEvent unwraps sandbox-SDK data.part.text natively (closes #35) #36).

Verification

pnpm exec tsx examples/sandbox-stream-backend/sandbox-stream-backend.ts

emits the canonical RuntimeStreamEvent sequence: task_start, readiness_*, session_created, backend_start, text_delta × 3 (the unwrap path #36 added), tool_call, tool_result, backend_end, task_end, final. Tested locally; CI runs pnpm run lint, pnpm test, pnpm typecheck — all green.