Add sampled session feedback survey

[comp615]

This PR description was generated by AMP.

Summary

This builds the generic sampled session survey on top of the response-feedback seam. It is stacked on the response-feedback PR and should merge after it.

The prompt becomes eligible after five visible user turns and ten minutes, uses a distribution-owned basis-point sampling rate, and applies a persistent 27–29 hour device-wide cooldown. Dismiss is the default focus and Escape dismisses only while the visible survey owns focus. Stock Berd still sends nothing; distributions own transport and rollout values.

Virtualized transcripts include the survey in offscreen height measurements without letting the hidden measurement copy focus or emit feedback.

Related issue

N/A — no duplicate issue or PR found. Depends on response-feedback PR #214.

Testing

bin/just check
bin/pnpm vitest run src/features/chat/response-feedback src/shared/runtime-config/schema.test.ts
bin/just _tauri-cargo-unix test --lib feedback_survey
bin/just tauri-check

AI tool validation prompt

Copy and paste into your AI tool:

Review block/berd branch ccroom/session-feedback-survey and verify:
1. Run the focused survey, runtime-config, and native cooldown tests.
2. Confirm sessions shorter than five visible user turns or ten minutes never claim a survey.
3. Confirm a visible prompt defaults focus to Dismiss, Escape dismisses only while it owns focus, and a session is never prompted again after an appearance.
4. Confirm the device-wide cooldown is persistent, atomic, and randomized from 27 through 29 hours.