[Streaming][P0] Pipeline DAV2 startup for sub-2-second first sound

[knzeng-e]

Outcome

Accept or reject the delivered DAV2 startup path from candidate-bound physical browser and device measurements.

Delivered

• Startup-phase telemetry and privacy-safe export tooling.
• Bounded header/first-range requests, pipelined read-ahead, worker decryption, and warm listening intent.
• Smaller adaptive first media range and explicit terminal failure handling.
• Browser regression coverage for interruption, autoplay, denied access, source replacement, and room handoff.

Remaining acceptance

• Collect cold and warm first-sound samples on Product Desktop, Safari/iPhone, Chrome, Firefox, and Android where available.
• Exercise broken/slow gateways and slow key-service responses.
• Report p75 for Free, already-authorized protected, and warm next-track playback using one exact candidate identity.
• Decide on backend read-through only from the measured bottleneck.

W13 owns the accepted candidate and go/hold decision. Design DAV3 only if these measurements show DAV2 is the limiting factor.

Local scope: docs/backlog/24-access-streaming-v2.md and docs/backlog/implementation/evidence/W08.md.