[REBASE] Add DWI Saving Support + Fix DWI Loading

[SilentMystification]

Rebased on beta + a little bit of cleanup

Summary

I have been making charts for Dance Maniax Update which uses the DWI format. ArrowVortex is my preferred step file editor and supports loading DWI files, but does not support saving DWI files.

Prior to this I had to go through an error prone multiprogram export to convert my ArrowVortex *.sm file to a *.dwi file.

This PR aims to change that. There are 3 major things implemented here:

Add DWI file saving support with full bracket coverage
Implements SaveDwi to serialize ArrowVortex simfiles to the DWI format.

Bug fix — LoadDwi {} quantization
{ was incorrectly assigned quantization 4 (48th notes). The correct value per DMX and StepMania 3.9 is 3 (64th notes, 1/64 * BEATS_PER_MEASURE). Any DWI file containing {} brackets previously loaded with notes at wrong positions.

Bug fix — LoadDwi closing bracket reset
All closing brackets (), ], }, ') reset the quantization to 24 (8th notes, 24 rows/slot). The previous value of 32 has no correspondence to any standard note division in ArrowVortex's 192-row system (192 ÷ 32 = 6 slots/measure) and would place subsequent bare notes at wrong row positions. The correct reset value matches the parser's own initial quantization: 192 / 8 = 24.

DWI Saving Details

Metadata
Writes #TITLE, #ARTIST, #GENRE, #FILE, #GAP, #BPM, #CHANGEBPM, #FREEZE, #SAMPLESTART, #SAMPLELENGTH, and #DISPLAYBPM. Timing offset is converted from AV's seconds-based offset to DWI's negated millisecond GAP. BPM changes and stops are written as comma-separated beat=value lists using DWI_beat = AV_row / 12.0.

Charts
Supports SINGLE, DOUBLE, COUPLE, and SOLO styles. Enforces DWI's one-chart-per-difficulty constraint with a warning on duplicates. Each chart header ends with a newline so note data starts on the following line.

Note data — quantization
Each measure selects the most compact bracket mode that covers all its notes:

Bracket	Grid	Rows/slot	Condition
none	8th	24	all rows divisible by 24
()	16th	12	all rows divisible by 12
[]	24th	8	all rows divisible by 8 (triplets)
{}	64th	3	all rows divisible by 3
`'	192nd	1	fallback

[] and {} cover disjoint note sets, so a measure mixing 24th and 64th notes falls back to 192nd.

Note encoding
Single note → numpad digit (4 2 8 6)
Two simultaneous notes → corner digit (7 1 9 3) or letter combo
3+ simultaneous → multi-note bracket
Hold heads: X!X or PairChar!PairChar for 1–2; <ABC!ABC> for 3+
One tap + one hold head → CornerChar!HoldChar without brackets (e.g. 7!4 for Up tap, Left hold)

[SilentMystification]

This may be personal preference but I thought this would be a useful default shortcut