[mattromano]

What

Closes #1090

Adds a Rock Band / Clone Hero / YARG drum chart importer, and an exporter for the other direction.

• lib/drumchart.py parses a song folder's drums part (notes.mid PART DRUMS or notes.chart, plus song.ini) with rules ported from YARG.Core: tom markers 110–112, 5-lane → 4-lane pro, disco flip, [ENABLE_CHART_DYNAMICS] accents/ghosts, 2x kick (95 / N 32), Easy/Medium/Hard, star power (116 / S 2), fills (120–124 / S 64), solos (103), sections, and the song.ini delay / .chart Offset. It encodes the result as drum_tab.json, using piece-ids from lib/drums.py (midi_to_piece, so it can't drift from the vocabulary the drums plugin renders) and the feedpak 1.20.0 gameplay keys (a, x2, star_power, fills, solos, levels). It also writes song_timeline.json (tempos, time signatures, beats, sections).
• lib/drumchart_gen.py covers charts that lack gameplay data. It reduces missing lower levels from Expert (rules fitted on 136 authored charts; marked generated), and generates YARG-style activation fills for charts with star power but no fills (SongChart.AutoGeneration).
• lib/sngfile.py reads .sng packages (SngFileFormat v1) in pure Python. numpy is test-only in core, so I rewrote the unmask as a 256-byte-period big-int XOR. It refuses directory components in member names and skips videos.
• scripts/ch_to_sloppak.py takes a folder or .sng and writes a pack with feedpak_version: 1.20.0: one type: drums arrangement pointing at drum_tab.json, the song-level drum_tab alias (spec §7.5 "Multiple drum parts"; arrangements may not be empty), song_timeline, the stems (lone song.ogg → full; CH stems → guitar/bass/drums/vocals/piano/other; crowd dropped), and the cover. A single .ogg per stem is copied verbatim; mixing or transcoding uses core's audio._ffmpeg_cmd().
• scripts/sloppak_to_ch.py loads a pack through sloppak.load_song (zip/dir, drum parts, song_timeline all handled by core) and writes PART DRUMS for the primary part or --part <id>. That includes the tempo map rebuilt from the beat grid (pickup / delay detection), tom markers, dynamics, Easy–Hard from levels, and SP/fill/solo phrase notes, plus EVENTS sections, song.ini and OGG stems.

Licensing: the parsing rules and auto_fills are ports of YARG.Core, which is LGPL-3.0 (not MIT — I checked its LICENSE). Its MoonscraperChartParser files are © Alexander Ong. lib/drumchart.py / lib/drumchart_gen.py carry the attribution and the note that LGPL-3.0 code may be combined into this AGPL-3.0 project. reduce_level and the exporter are original.

No print() in lib/ (the scripts log through logging, like scripts/lift_keys_notation.py). No JS changes.

feedpak surface

• This PR does not change how the app reads/writes feedpaks (manifest keys, pack files, folder layout)
• …or it does, and the spec change landed first via the FEP process — FEP / spec PR: got-feedback/feedpak-spec#68 (once it merges, re-run this PR's checks and the gate goes green)

Dependency: stacked on got-feedBack/feedpak-spec#68 (FEP got-feedBack/feedpak-spec#67). The drum_tab keys this writes (hits[].a, hits[].x2, star_power, fills, solos, levels) are defined there. Please merge that first. All manifest keys written (feedpak_version, title, artist, album, year, duration, cover, stems, arrangements with type/drum_tab, drum_tab, song_timeline) already exist in spec main, and the gate only scans top-level manifest keys. So the feedpak-spec CI job passes today against spec main as well as the spec branch (verified locally, all 4 layers). It won't mechanically block. Holding the merge for the spec PR is a process rule, not a CI one. feedpak-spec-exceptions.yml isn't involved and can't be used here: CI fails any PR that adds to it, and there's no in-repo bypass (docs/feedpak-spec-gate.md).

Core reads nothing new. Streaming the gameplay keys over the highway WS to a drums renderer is a follow-up: today drums.hits_to_wire forwards t/p/v/g/f/k only.

Checklist

• CHANGELOG.md [Unreleased] updated (user-visible changes)
• Tests added/updated for new behaviour
• Commits are DCO signed off (git commit -s)

Tests: tests/test_drumchart.py (26) and tests/test_sloppak_to_ch.py (5), with fixtures in tests/drum_fixtures.py. They use synthetic MIDI/.chart/.sng files and stand-in OGG bytes, so no ffmpeg or audio is needed in CI. Coverage: tempo maps, tom markers, dynamics, 5-lane, disco flip, delay/offset, lower levels, encoding (every pad ↔ piece, x2 only at the top level, empty metadata omitted, payload passes drums.validate_drum_tab and hits_to_wire), reduction rules, auto fills, folder → pack → sloppak.load_song (a drum part, a placeholder-only arrangement list so nothing is graded as fretted, timeline loaded), CH stem mapping, .sng input, CLI errors, a CH → pack → CH round trip (hits, levels, SP/fills/solos, sections, beats within 2 ms; second conversion identical), part selection, unknown pieces, pickup handling, and .sng unmask against a byte-wise reference plus path-traversal stripping.

• pytest tests/test_drumchart.py tests/test_sloppak_to_ch.py plus the existing drums tests (test_drums_lib, test_sloppak_drum_parts, test_sloppak_drumtab_load, test_midi_import_drums, test_spec_gate): 108 passed.
• Full suite on Windows: 2789 passed, 10 failed. All 10 are the known Windows-only failures (symlink / feedpak_extension scan / tailwind / plugin asset / db-restore); none touch this change.
• tools/check_spec_conformance.py --spec <feedpak-spec main> and --spec <spec branch>: OK.
• A converter-produced pack (zip and dir form) validates with the spec branch's tools/validate.py: PASS.

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📒 Files selected for processing: 9

• CHANGELOG.md
• lib/drumchart.py
• lib/drumchart_gen.py
• lib/sngfile.py
• scripts/ch_to_sloppak.py
• scripts/sloppak_to_ch.py
• tests/drum_fixtures.py
• tests/test_drumchart.py
• tests/test_sloppak_to_ch.py

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[mattromano]

Marking ready: this is part of the drum gameplay metadata chain (feedpak-spec#68 → feedBack#1105 streams it → feedBack#1091 imports it) that got-feedBack/feedBack-plugin-drums#25 uses for star power, fills, solos and per-difficulty levels.