fix(rendering): rhythm stems for slashed beats on tab staves

[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 #2888

Proposed changes

Rendering a tab staff that contains slashed beats crashed whenever tab rhythm stems were enabled (ShowWithBars or ShowWithBeams).

Root cause: TabBeatGlyph draws slashed beats as a SlashNoteHeadGlyph instead of fret numbers. For those beats:

• it reserved no collision slot, so TabBarRenderer.paintBeamingStem crashed on an empty slot list;
• every Y query returned 0, which put stems, beams and augmentation dots at the top of the staff.

Fixes

• Slashes on tab staves reserve their area. They resolve Y positions and attach the stem at the SMuFL stem anchor, as on the standard notation and slash staves.
• A slashed beat that contains tied notes is now rendered as a slash on tab staves too. Before, it fell back to fret numbers while standard notation showed a slash, and the hidden tied note pulled the stem up to its string.
• A tremolo on a slashed tab beat sits on the stem, which matches Guitar Pro. If the stem is hidden, the tremolo still sits where the stem would be. Whole-note slashes keep it centred.

Refactoring
Done to avoid adding another copy of the position logic:

• NoteHeadGlyphBase provides the note head positions and stem attachment points from the SMuFL anchors. Standard notation chords, the slash staff and tab slashes use them instead of separate copies. The stem end positions stay specific to each staff type.
• ScoreBeatGlyph, SlashBeatGlyph and TabBeatGlyph resolve rest positions through one shared helper. This also fixes tab rests resolving Bottom to their top.
• The visual suite shows no pixel differences from the refactoring.

Known limitation, not addressed here: on tab staves, a hidden tied note pulls the rhythm stem up to its string in normal chords too. That is independent of slashes and should be handled separately.

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

New visual tests GuitarTabs/rhythm-slashed and GuitarTabs/rhythm-with-beams-slashed render beat-slash.gp with both rhythm modes. The beat-slash reference changes because of the tied slashed beat in bar 5.

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