feat: osu!mania -> quaver reverse conversion (+ combo/positioning fixes)

[dromzeh]

closes #2, closes #3, closes #4

three for one: both open fixes plus the reverse direction. the fun part - the gist formulas are the osu!->quaver direction already (the existing converters are their inverses), so the reverse coordinate math was basically free and round-trips exactly (there's a test: ComboPosY -> ComboPosition -> ComboPosY is lossless).

the fixes

• combo colour during LNs (ln combo colour changes instead of static consistent #3) - osu! tints the combo counter with ColourHold (default orange 255,191,51) while a hold is pressed; quaver never touches it. every [Mania] block now writes a white ColourHold so it stays static.
• perfect/combo positioning (perfect/combo values positioning not exact 1:1 #2) - turned out to be a defaults bug, not a formula bug. checked the math against lazer's ManiaLegacySkinTransformer + LegacyManiaJudgementPiece (both centre-anchored at N*1.6 from screen top) and quaver's GameplayPlayfieldKeysStage (MidCenter + offset) - the existing gist-inverse formulas are already exact. the real issue: a skin.ini that omits ComboPosY/JudgementBurstPosY inherits the default skin's values in quaver (-40 / 108 in bar, arrow and circle alike), and we assumed 0 - judgements ~68px and combo ~25px off in osu! space for those skins.

how the reverse works

• detection - auto-detected from the input's skin.ini: [Mania] blocks = osu! skin, [4K]/[7K] = quaver. .osk in, .qs out (folder + archive, --open installs in quaver).
• skin.ini - new osu! parser (Key: Value, repeated [Mania] blocks keyed by Keys, // comments) and a quaver Key = Value writer. [Mania] geometry maps to [4K]/[7K] via the gist's forward formulas.
• assets - per-column notes/holds/receptors honouring skin.ini overrides, osu!'s default mania-note1/2/S per-column styles, and @2x fallbacks (halved back to @1x metrics). LN tails flip back, health bars rotate back to vertical, lighting animations get recomposed into quaver's @1xN spritesheets, judgements/fonts/grades/hitsounds all map back.
• receptors - written ColumnSize wide at their osu! pixel height, so quaver's aspect scaling reproduces osu!'s drawn size - which also gives an exact HitPosOffsetY back out of HitPosition.

notes

• stage hint is copied unchanged: osu! centres it on the hit line, quaver bottom-anchors it, so content below the line can't be represented - flagged in the report instead of silently cropping.
• animated hold bodies take frame 0, hitsounds are .wav-only - both match what the forward direction already does.
• osu!-only files (standard mode, menus, ...) are skipped and counted in the report.

test

• go test ./... - osu! parse test, reverse coordinate goldens + round-trip, and a reverse end-to-end over a synthetic .osk (overrides + default filenames, flipped tail, rotated health, sheet sizes)
• built the exe and round-tripped a skin both directions: direction auto-detected, ComboPosY -120 / JudgementBurstPosY -85 came back exactly
• not yet eyeballed in-game (parsec) - worth a look before merge

[dromzeh]

so sorry to the people i accidentally @'d my bad guys