perf(#270): artwork base64 が未変化ならデコード済み Data を再利用

[GeneralD]

Closes #270 (Part of #258)

概要

media-remote-helper はイベント毎にフルペイロードを再送するため、同一トラックの elapsed 更新でも高解像度アートワークの base64 フルデコードが毎回走っていた。直前のイベントの base64 文字列と比較し、一致時はデコード済み Data を再利用するメモ化を入れて、変化時のみデコードするようにした。

変更内容

• MediaRemoteDataSourceImpl.artworkData(from:) を追加 — StreamStateBox に lastArtworkBase64 / lastArtworkData を持たせ、lock 保護下で比較・キャッシュする
• デコード関数 decodeBase64 を init 注入に変更(public convenience init() は従来どおり Data(base64Encoded:) を使用)— テストでデコード呼び出し回数を検証可能にするため
• テスト 3 本追加:
  • 同一 base64 が連続 → デコードは 1 回のみ、両イベントとも同じ Data を返す
  • base64 が変化 → 再デコードされる
  • artwork なしペイロード → デコーダは呼ばれない

補足

• 文字列比較は O(n) だが、Data アロケーション + base64 デコード本体に比べ十分軽量
• media-remote-helper のアートワーク送出を楽曲変更時のみに限定する #255(helper 側で artwork 送出を楽曲変更時のみに限定)と両輪。helper 側が対応しても、旧 helper との互換やレースに対する daemon 側ガードとして有効

テスト

• swift test 全 928 件パス
• make format 適用済み

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📥 Commits

Reviewing files that changed from the base of the PR and between 70564c9 and e7b7c50.

📒 Files selected for processing (3)

• Sources/MediaRemoteDataSource/MediaRemoteDataSourceImpl.swift
• Sources/VersionHandler/Resources/version.txt
• Tests/MediaRemoteDataSourceTests/MediaRemoteDataSourceImplTests.swift

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• Create PR with unit tests
• Commit unit tests in branch perf/#270-artwork-base64-decode-cache

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