test: unblock progress/runner coverage and extend updater/prompt tests

[DecampsRenan]

Summary

Addresses the three test-coverage gaps called out as follow-ups: the updater HTTP path, the prompt success paths, and the progress/runner packages that couldn't be tested because of their transitive ALSA dependency.

Updater — inject Downloader interface

• New Downloader interface and HTTPDownloader default implementation.
• Execute() now takes a Downloader, so tests can feed canned archive bytes or network errors without hitting the network.
• Tests added: homebrew refusal, download error propagation, invalid archive, missing binary in archive, successful replace, missing target, plus HTTPDownloader success / non-200 / network error paths via httptest.
• Coverage: ~47% → 72.6%.

Audio — split CGO playback into its own package

• Moved the beep/oto code (Player, PlayMusicAndWait, PlaySound, embedded .mp3 files) from internal/audio into a new internal/audio/playback package.
• internal/audio now only contains the pure-Go launchers (StartVibes, PlayNotification, NotificationSound) — importing it no longer pulls in ALSA.
• cmd/root.go wires the hidden _play-music / _play-sound subcommands to the new playback package.
• This unblocks progress and runner tests in environments where CGO is enabled but libasound isn't installed.

Progress — new model tests

• Added internal/progress/model_test.go covering the ring buffer (addLine), the Update state transitions (output line / done / timer / window size), the three View render modes (running / success / failure / truncation), and listenCh channel behavior.
• Coverage: 0% → 38.1%.

Prompt — extract pure helpers, test them

• Extracted findDetectionByPM, truncateCmd, and scriptOptionLabel from the TTY-bound flows so they can be unit-tested without a PTY.
• Select and SelectScript now delegate to those helpers; behavior is identical.
• Coverage: 13.3% → 26.7%.

Test plan

• CGO_ENABLED=0 go test ./... — all green
• go test ./internal/audio ./internal/progress ./internal/runner ./internal/prompt ./internal/updater with default CGO — all green (previously progress/runner failed to build)
• Verify on macOS that _play-music and _play-sound still work end-to-end after the audio package split
• Verify spm upgrade still downloads a release through HTTPDownloader