fix(rendering): correct the minimum bar spacing and hammer-on/slide slur placement

[Danielku15]

Note

AI-authored disclosure (alphatab-ai-authored-v1)

Portions of this content were authored by an AI agent. The agent has read
AGENTS.md and the human submitter accepts responsibility for
compliance with the rules in that document.

Issues

Fixes #2904

Proposed changes

#2904 reports a slur with NaN control points between two legato-slide chords at narrow widths. The root cause
is not the slur: the bar gets far less width than its content needs (258px for a measured minimum of 809px at a
400px container), every spring is compressed linearly, the notation overlaps and the slur end anchor lands
before its start anchor. Looking into this showed that the minimum itself was wrong in two ways, and that the
effect slurs were anchored at positions which collide as soon as space gets tight.

Spacing minimum (BarLayoutingInfo)

• The first beat's pre-beat width (the start-of-bar spacing for accidentals etc.) was counted twice in the
  minimum stretch force. A single accidental on the first beat inflated every beat of the bar (the Slur of a legato slide / hammer-on between two-note chords is not drawn in the standard notation staff when beats are tightly spaced (path with NaN) #2904 bar
  needed 809px instead of 451px). It is a fixed offset in front of the springs and is no longer stretched in.
• The minimum only reserved the ink of adjacent beats: at the minimum spacing an accidental or grace group
  touched the previous notehead and flags touched the bar line. Two new engraving settings add the clearances
  Behind Bars asks for, only where content would come closer:
  • EngravingSettings.beatContentPadding = ½ stave-space between adjacent beats (Behind Bars p. 41), including
    trailing grace groups.
  • EngravingSettings.barlineContentPadding = 1 stave-space between the last beat and the bar line (p. 43).
• Rhythmic spacing is unchanged: all beats of a bar still share one force, so notes of equal duration stay
  equally spaced (p. 40). Bars which were only wide because of the double count get narrower, bars with
  colliding content get the room they need.
• Multi-bar rests are now centered in their bar (p. 564) instead of keeping the extra width on the right.

Squeezed bars

• Ties and slurs keep a minimum span (the beat content padding), centered on their anchors. Anchors which
  crossed in a squeezed bar produced the NaN path (log of a negative span) or an inverted curve.
• When bars still have to be compressed below their minimum, the layout logs one warning per layout pass, e.g.
  Bar 1 does not fit into the available width and had to be compressed below the minimum size (up to 311px missing).
  No public API for the required width yet; that needs a proper design first.

Hammer-on, pull-off and legato slide slurs (cross-checked against Guitar Pro, MuseScore and Behind Bars)

• Standard notation: the slur started at whichever note of a chord was processed first, often an inner note,
  and ran from that notehead to the accidentals of the next chord (the slur of Slur of a legato slide / hammer-on between two-note chords is not drawn in the standard notation staff when beats are tightly spaced (path with NaN) #2904). All notes on one stem
  now take a single slur outside the chord, connecting the outer noteheads (p. 114). It spans from the beat
  where note chains start to their furthest destination, so a chain continuing across a beat on another
  string stays one slur.
• In multi-voice bars the slur goes to the stem end of its voice (p. 117: never on the notehead side in
  double-stemmed writing), so the slurs of two voices no longer collide between them.
• Tab: the arcs of a chord were merged by comparing against the tie direction before it was computed. Lower
  string arcs were merged into the upper arc and lost, arcs on the lower strings were never merged. A beat now
  gets one arc per side (above the upper strings, below the lower strings) from the outer note to the furthest
  chain end, matching Guitar Pro. Numbered notation keeps its previous behavior.

Tab staff lines

• The staff line gaps behind fret numbers baked the note position into a cache that was not updated when the
  notes moved afterwards (centered full-bar notes, shared pre-beat sizes). Lines were cut next to the numbers
  and ran through them. The gaps now read the current position.

Not in this PR

• The H/P/sl. labels still sit on the arcs. Guitar Pro and MuseScore place them above the staff, stacked once
  per chord segment; that is a larger layout change and will be explored separately.
• Overflow handling for bars which cannot fit at all (overflow/scale modes, splitting bars across systems).

Tests

• Regression test for the Slur of a legato slide / hammer-on between two-note chords is not drawn in the standard notation staff when beats are tightly spaced (path with NaN) #2904 snippet at 400px and 300px (no NaN in the SVG output) and for the warning.
• Unit tests for the spacing minimum (no first pre-width, beat padding, bar line padding).
• New visual tests for effect slurs on chords (accidentals, stems up, opposite stems, different chain lengths,
  mixed kinds, ties, chains across another string) and in multi-voice bars (plain and beamed).
• Reference images updated for the spacing, slur and staff line changes.
• C# and Kotlin ports build, the C# test suite passes.

Checklist

• I consent that this change becomes part of alphaTab under its current or any future open source license
• This PR is linked to an accepted issue (see above)
• Changes are implemented
• New tests were added
• I have read AGENTS.md if an AI helped draft any part of this PR

AI authorship disclosure

• No AI agent authored any part of this PR (description, code, tests, or commit messages)
• An AI agent contributed to this PR. The AI-authored disclosure block
  (alphatab-ai-authored-v1) is present at the top of this body, and I have
  personally reviewed every change and can explain each one

Further details

• This is a breaking change
• This change will require update of the documentation/website