Fix stalled first-run model downloads

[r3dbars]

Summary

• replace the fake 8% floor with FluidAudio's real byte-level model progress
• fail downloads after five minutes with no upstream activity and expose the existing Retry Download path
• make timeout/retry ownership generation-safe so abandoned work cannot overwrite a newer attempt
• report a bounded privacy-safe parakeet.model_download_stalled Sentry event

User impact

A first-run model download can no longer sit on an unchanging progress indicator indefinitely. Active slow downloads remain alive while callbacks arrive; truly stalled downloads become an actionable failure with retry copy.

Verification

• TRANSCRIPTED_DISABLE_FILE_LOGGER=1 bash run-tests.sh — 13,413 passed
• TRANSCRIPTED_DISABLE_FILE_LOGGER=1 bash build.sh --no-open — signed app, signature valid, launch smoke passed (953.2ms)
• git diff --check
• independent thermonuclear review of the full diff
• final independent review after fixing callback-cycle/liveness handling — PASS

Review findings fixed

• Retry now detaches the canceled initialization instead of rejoining it.
• Late completion from an abandoned prefetch cannot clear or overwrite a newer retry.
• FluidAudio's one-listing/four-component callback sequence maps monotonically to 0–100%.
• Every callback refreshes liveness even when UI updates are throttled.
• The Sentry event uses error level so the explicit allowlist can forward it.

Proof boundary

Deterministic tests and signed local build are green. A real throttled or interrupted first-run network download on a separate Mac remains manual/unknown until tested. No release, appcast, Homebrew, notarization, or publishing change is included.