feat(tags): Add multi-artist and multi-genre tag splitting

[SteveZMTstudios]

Summary

Tracks whose artist or genre tag holds several values (A; B, A & B, A feat. B) currently land in the library as a single artist with that whole string as its name, so they never show up under either artist. This adds opt-in splitting so they are indexed under each value separately.

Both switches default to off, so an existing library is unaffected until the user turns them on and the library is rescanned.

What it does

• Tag Splitting category in Behavior settings with two switches, Multi-artist support and Multi-genre support, each with its own delimiter page underneath. A delimiter page stays disabled while its switch is off.
• Delimiter pages (SplitTagSettingsActivity) edit symbols and words as dismissible chips, validate as you type (single character, no letters or digits, no backslash, no whitespace in words, no duplicates), and offer Reset to default from the toolbar.
• Artist navigation stops guessing. When a track has several artists, tapping the artist opens a picker; otherwise it goes straight to the artist page. Resolution is id → exact name → case-insensitive name, so entries reached by name (Android Auto, media ids like artist_<name>) still land on the right page when the MediaStore id is not available.
• Search matches title, album, artist and genre through one shared predicate.
• Play queue persists the new extras and the raw tag, so a restored queue keeps showing what it showed before.

How it works

Tokenizer — TagSplitter is the single place that knows about delimiters. Symbol delimiters match anywhere, word delimiters only when surrounded by whitespace ((?<=\s)(?:feat\.|ft\.)(?=\s)), so feat. inside a name is left alone. A backslash before a delimiter escapes it: escaped delimiters are swapped for placeholders before matching and restored afterwards. Splitting and display formatting happen in one pass, and the display string is normalized onto the primary delimiter so A & B and A; B render the same way.

Artist aggregation — Reader.read now builds artists in two passes. The first pass fills a canonical (trimmed, lowercased) name → MediaStore artist id cache from all songs; the second maps each song onto its artists against that finished cache (MiscUtils.aggregateArtistsInto, a pure function). The order is load-bearing: resolving during the scan would make results depend on which song was read first.

Album artists — MiscUtils.resolveAlbumArtist takes the album-artist tag by majority (60% of tracks, ties broken against null, with a warning logged when there is no clear winner) and otherwise infers the album artist from the track artist tags. Artists without a MediaStore id get a deterministic negative synthetic id derived from the canonical name.

Plumbing — MediaItemExtras exposes rawArtist, artistNames and genreNames on top of EXTRA_RAW_ARTIST, EXTRA_ARTIST_NAMES and EXTRA_GENRE_NAMES; each falls back to the unsplit value, so callers that never see a split item keep working. GramophoneApplication publishes a tagSplitConfigFlow that re-runs the reader, debounced so rapid chip edits do not trigger a rescan storm — but the first emission is not delayed, so cold start is unaffected.

Defaults and trade-offs

• Default delimiters are ;, ， and 、, the separators used by mp3tag, MusicBrainz Picard and most streaming exports.
• ASCII ,, & and / are not in the default set on purpose: they occur inside plenty of single-entity names (Tyler, The Creator, Earth, Wind & Fire, AC/DC, Above & Beyond, Simon & Garfunkel). Users who want them can add them.
• A name that would be split can be protected by editing the tag and escaping the character (AC\/DC). This is the only escape hatch and it is a tag edit, not a per-file app setting — that is a deliberate choice, not an oversight.
• Enabling a delimiter that really does occur inside a name means a fragment can resolve onto a different artist that happens to share that name. The escape above is the intended remedy.

Scope notes

• Search additionally matches genre now, including with multi-genre off (the fallback is the unsplit genre tag), so the search surface grows slightly on the default path.
• Artist entries are now keyed on the canonical name with the MediaStore id as fallback, so names that differ only in case or surrounding whitespace merge into one entry.
• app/src/main/res/layout/preference_basic.xml is a shared layout (~28 references across the settings screens). It now points at two new colour state lists, which is what lets a dependent preference grey out. The selectors' enabled state is byte-identical to the previous values, so already-enabled preferences render exactly as before.
• isColorOS() gained ro.build.version.realmeui, which keeps StrictMode off on realme UI devices; that behaviour was already intended for ColorOS/OPPO builds.

No new permissions, no manifest components other than the settings activity (exported=false), no database migration, no change to minSdk.

Testing

./gradlew :app:testDebugUnitTest — 24 new tests in TagSplitterTest:

• splitting on symbols, on words, and on CJK/ASCII mixed input
• single-entity names (AC/DC, Above & Beyond, Earth, Wind & Fire, …) left intact under the defaults
• backslash escaping, including words
• symbol/word normalization and deduplication
• synthetic ids are strictly negative and stable across casing
• album-artist majority fallback
• two-pass aggregation against real MediaStore ids

Checklist

• New strings added to the default values/strings.xml
• Feature is opt-in and off by default
• ./gradlew :app:assembleDebug
• ./gradlew :app:testDebugUnitTest
• Verified on a device with a multi-artist library

Screenshots

Artist selector	per artist	srlit config