Add CodSpeed performance benchmarks

[codspeed-hq]

Summary

This PR sets up continuous performance measurement with CodSpeed for downcloud.

The project is a Bun + TypeScript CLI whose hot paths are mostly I/O (network, ffmpeg), but it contains several CPU-bound helpers that are good candidates for tracking: SoundCloud client_id discovery (regex scanning of homepage HTML and minified asset bundles) and waveform processing (downsampling, normalizing, and ASCII rendering of amplitude samples).

To benchmark this logic without importing the Bun-specific CLI entrypoint, the pure functions were extracted into a new lib.ts module, and index.ts was refactored to consume them. This keeps a single source of truth so the benchmarks track the real code.

Changes

• lib.ts (new): Pure, side-effect-free helpers extracted from index.ts — extractAssetUrls, findClientIdInChunk, compressWaveform, normalizeWaveform, renderAsciiWaveform, originalArtworkUrl, and formatTime.
• index.ts: Refactored to import and use the helpers from lib.ts. Behavior is unchanged (verified the CLI help and type-checking still pass).
• bench/lib.bench.ts (new): Vitest benchmarks covering client_id discovery, waveform processing (1800 and 20000 sample inputs), and metadata helpers, using realistically sized synthetic inputs.
• vitest.config.ts (new): Registers the @codspeed/vitest-plugin.
• package.json / bun.lock / bun.nix: Added vitest and @codspeed/vitest-plugin as dev dependencies.
• .github/workflows/codspeed.yml (new): GitHub Actions workflow that installs Bun, installs dependencies, and runs the benchmarks through the CodSpeed action in simulation mode using OIDC authentication.
• README.md: Added the CodSpeed badge.

Local verification

All 8 benchmarks run successfully under CodSpeed simulation mode locally (codspeed run --mode simulation -- bunx vitest bench --run), and results upload and link to the yaaaarn/downcloud repository.

Next steps

• Merge this PR so the baseline is recorded on master.
• Subsequent pull requests will automatically get performance reports comparing against the baseline.
• Consider adding more benchmarks as new CPU-bound logic is introduced.

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 8 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• extractAssetUrls (homepage HTML): 278.1 µs
• findClientIdInChunk (minified asset bundle): 347.7 µs
• formatTime: 31.2 µs
• originalArtworkUrl: 26.9 µs
• compressWaveform (1800 samples): 264.1 µs
• normalizeWaveform (1800 samples): 289.5 µs
• renderAsciiWaveform (1800 samples): 338.9 µs
• renderAsciiWaveform (20000 samples): 1.8 ms

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