Add ace-understand: analyze audio via local path or URL, dogfood in CI

[Copilot]

Extends the action with a reverse-pipeline mode using ace-understand — given an audio file, the action extracts caption, lyrics, BPM, key, duration, and language as JSON. The understand input accepts either a local file path (used directly) or a URL (http/https/ftp/file://, downloaded via curl).

Action interface

• New understand input (string, default empty) — activates understand mode, skips generation
• New understand_result output — multiline JSON from ace-understand

Entrypoint (src/entrypoint.sh)

• Early-exit branch when INPUT_UNDERSTAND is set; detects URL vs local path via scheme prefix
• Local path: validated (exists + non-empty), passed directly to --src-audio
• URL: downloaded with curl -fsSL --max-time 300, validated, then passed to --src-audio
• Output written to $GITHUB_OUTPUT using a bash-native heredoc delimiter ($$_${RANDOM})

Dockerfile

• Fixes typo ace-undestand → ace-understand (binary was never being copied)
• Adds curl to apt dependencies

CI (test.yml)

• test-understand job depends on test (needs: test), downloads the generated WAV artifact into $GITHUB_WORKSPACE, and passes the container-side path /github/workspace/output.wav directly — no external audio, no file:// workaround
• Validates output is valid JSON with caption, lyrics, and bpm fields

Usage

# From a URL
- uses: audiohacking/acestep-action@main
  with:
    understand: 'https://example.com/song.mp3'

# From a local path (e.g. file already in the container workspace)
- uses: audiohacking/acestep-action@main
  with:
    understand: '/github/workspace/output.wav'

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