music indexer: add disc number to each track (from ID3 TPOS)

ffprobe surfaces ID3 TPOS as the `disc` tag ("n" or "n/total"); parse the leading
number → IndexTrack.disc?: number in meta.json (undefined when absent/unparseable),
alongside `track`. Verified on Pink Floyd - The Wall (Disc 1 → 1, Disc 2 → 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/sidecar/music/indexer.ts

@@ -9,7 +9,7 @@ import { homedir } from 'node:os';
 // (relative to the Music root):
 //
 //   DATA_PATH/music/cache/<rel>/meta.json   { path, cover?, tracks: [{ file, title, artist, albumArtist,
-//                                             album, track, year, durationSec, lyrics?:'lrc'|'txt' }],
+//                                             album, track, disc?, year, durationSec, lyrics?:'lrc'|'txt' }],
 //                                             videos?: [{ file, title, durationSec, width, height, poster? }],
 //                                             images?: [{ file }] }   (phone IndexMeta schema)
 //   DATA_PATH/music/cache/<rel>/cover.jpg    compressed (<=600px, jpeg q5)
@@ -89,6 +89,7 @@ export type IndexTrack = {
   albumArtist?: string;
   album?: string;
   track?: string;
+  disc?: number; // disc / part-of-set number, from ID3 TPOS (the `disc` tag "n" or "n/total")
   year?: string;
   durationSec?: number;
   lyrics?: 'lrc' | 'txt'; // available lyrics format ('lrc' = synced/timestamped); text via /lyrics. Absent = none.
@@ -344,6 +345,8 @@ async function ffprobeTrack(absPath: string, file: string): Promise<{ track: Ind
     for (const [k, val] of Object.entries(fmt.tags ?? {})) tags[k.toLowerCase()] = val;
     const durNum = fmt.duration ? Math.round(parseFloat(fmt.duration)) : undefined;
     const yearMatch = tags.date?.match(/\d{4}/);
+    // Disc / part-of-set from ID3 TPOS (ffprobe surfaces it as `disc`, "n" or "n/total").
+    const discNum = tags.disc ? parseInt(tags.disc.split('/')[0]!.trim(), 10) : NaN;
     // Embedded lyrics: USLT/foobar keys — `lyrics`, `lyrics-<lang>`, `unsyncedlyrics`.
     let lyricsText: string | undefined;
     for (const [k, val] of Object.entries(tags)) {
@@ -360,6 +363,7 @@ async function ffprobeTrack(absPath: string, file: string): Promise<{ track: Ind
         albumArtist: tags.album_artist || tags.albumartist || undefined,
         album: tags.album || undefined,
         track: tags.track || undefined,
+        disc: Number.isFinite(discNum) ? discNum : undefined,
         year: yearMatch ? yearMatch[0] : undefined,
         durationSec: Number.isFinite(durNum) ? durNum : undefined,
       },