test: rust daemon unit test coverage

[CougarTasker]

Features

• Adds unit-test coverage across the Rust daemon: the cloud coordinators (uploader, status, progress, registration), the recording_reaper watcher, the pipeline (dispatcher, json_writer, trace_actor), the API client's retry/auth paths, and the shared env-config layer.
• Producer video spool is now lossless PNG, not raw RGB24. Each frame is PNG-encoded (truecolour, 8-bit, "Up" filter, deflate level 1) on the writer/drain thread before it hits disk, so the on-disk .nut — and the daemon's transcode read-back of it — is a fraction of the raw video bandwidth. Chunk rolls key off decoded-equivalent ("logical") bytes so per-chunk transcode granularity is unchanged regardless of compression ratio. Buffers and the zlib compressor are reused per writer, so a steady-state frame allocates nothing. Adds an ffmpeg-backed bit-exact round-trip test.
• Minor test-enablement refactors: the upload-chunk backoff cap now reads the API client's configurable max_backoff (lets retry/stall tests run without real-time sleeps), and NDD_DEBUG parsing now shares the common truthy set.

Bugfixes

• None

Items

• NCP-984

Related PRs

• Stacked Rust daemon release — review/merge in order:
  1. feat: scaffold the rust data daemon #718 — workspace + CI + docs
  2. feat: integrate rust daemon behind a flag #719 — Python integration + wheel packaging (dark, default-off)
  3. test: rust daemon integration suite #720 — integration tests + CI matrix
  4. test: rust daemon unit test coverage #734 — unit-test coverage + producer PNG video spool (this PR)

[damonhayhurst-neuraco]

LGTM