feat(rendering): place hammer-on, pull-off and slide labels above the staff and improve effect annotation spacing

[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 #2944

Proposed changes

H / P / sl. labels as effect bands above the staff

The labels were painted on the slur arcs, which put them below the tab staff for lower arcs, lost labels when a
chord mixed kinds and showed a label per arc. Like Guitar Pro and MuseScore they are now annotations above the
staff for standard notation, tab and numbered notation:

• One effect band per kind (hammer-on/pull-off, legato slide), both built from the same implementation. The
  existing EffectHammerOnPullOffText / EffectSlideText notation elements keep controlling their visibility.
• One label per beat for all notes of the chord, centered between the beats of the segment. Different texts of
  one kind (H on one string, P on another) and different kinds ("sl." above "H") are stacked with the usual band
  padding.
• Segments crossing a bar line are centered across it. Segments crossing a system break are labeled on the first
  system, centered up to the system end; the continuation gets no label.
• Labels are derived from the model (each note now knows the slur segment starting at it), not from the arc
  glyphs. The label code of the tie/slur glyphs is removed, which also stops regular slurs from showing effect
  labels.

Effect band placement and spacing

• A bar now reserves its own content plus the bands placed on it, taken from their final position. Before, the
  reserved height was derived from the skyline growth during placement, which also contained content of other
  bars (ties, brackets) and missed content registered only as overflow. Bands could be cut at the top of a
  system or leave too much space (e.g. sustain pedal markings). The height is now rounded once instead of twice,
  which moves many references by 1–2px.
• Note-attached markers (labels, tap, pick stroke, harmonics, fingering, …) are placed glyph by glyph instead of
  as one block per bar, so markers of different effects can share a row in the gaps between each other. All
  glyphs of a band still share one height.
• A glyph can keep a wider range clear than it occupies: labels keep their whole segment clear (so they stay above
  fret numbers and noteheads of the segment), markers attached to a beat keep the noteheads and stems of their
  own beat clear (e.g. a pick stroke above the stem of its note).
• The minimum padding between the content of adjacent staves also considers content horizontally closer than
  ½ stave-space (Behind Bars), not only content at the same x position.
• Jump direction texts (Fine, To Coda, …) are drawn from the top like other text effects; centered on the font's
  middle line their ink reached above the reserved space.

Known limitations (not addressed here)

• Text effects report the advance width of their text as bounding box, not the ink. With the horizontal staff
  padding this occasionally adds a stave-space where the ink of two annotations is just far enough apart.
• In the horizontal layout, a label centered exactly on a partial boundary is cut, like the slur arcs already are.

Tests

• New visual tests for effect slur labels on chords and in multi-voice bars, across bar lines and across system
  breaks.
• Reference images updated for the label placement and the spacing 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