fix(rendering): start continued ties/slurs after clef on new systems

[GiovanniSlabs]

When a tie or slur continues on a new system, the curve segment started at the
very left edge of the staff, overlapping the clef and key signature. This PR
anchors the continuation start at the first bar renderer's beatGlyphsStart,
so it begins after the clef, key and time signatures as in standard engraving
practice.

Issues

Fixes #2799

Proposed changes

• The final segment of a cross-system tie/slur (TieGlyph.doLayout,
  continuation branch) now starts at the end staff's first bar renderer
  beatGlyphsStart instead of the staff left edge. The anchor renderer was
  also switched from the start staff to the end staff, since the segment is
  painted on the end system.
• The full-system segments of multi-system slurs
  (ContinuationTieGlyph.calculateStartX) use the same anchor.
• Ties ending on the first beat of a system have their end X directly at the
  start of the beat area, so the continuation stub keeps a minimum length of
  2 staff spaces (smuflMetrics.oneStaffSpace) to stay visible. Happy to
  adjust if you prefer a different metric or approach for this minimum.
• The 9 affected visual test references were regenerated via
  npm run test-accept-reference and visually reviewed; a new visual test
  (multi-system-tie-first-beat) covers the tie-ending-on-first-beat case.

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• New tests were added

Further details

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