fix(bend): GP8 short-bend curve loss + 2D curve timing + 3D bnv gating (#535)

Post-merge Codex review of the bend-curve PRs (#531/#532) surfaced edge cases:

- GP8 (#531 P2): bnv timing used rn.sustain, which is zeroed for notes <= 0.2s,
  so short GP8 bends kept the scalar bn but lost bt/bnv. Use the beat duration
  `dur` (matching the GP5 path) so the curve survives.
- 2D highway (#532 P2): bnvNormalizedPoints mapped x over the curve's own t-range
  [first,last] instead of the note span, so curves not starting at 0 / ending at
  sus were time-distorted. Now maps over [0, sus] (clamped), with a curve-span
  fallback when sus<=0 (existing no-sus callers unaffected).
- 3D highway (#532 P3): the sustain ribbon + bend chevron were gated on bn>0, so a
  note carrying an authoritative bnv with bn==0 drew no ribbon/marker. Both now
  also fire on bnv presence; chevron steps derived from max(bn, bnv peak).

Codex-reviewed: clean (no findings). +1 JS test (sus-relative mapping + fallback).
JS 8/8, 250 core GP/song tests pass.

NB: GP8's short-bend path still lacks a dedicated synthetic-GPIF fixture (same gap
as the GP8 offset-prop-names P3) — _gpx_bend_shape units cover the function; the
fix is the one-line caller change.

Part of got-feedback/feedback#334.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/gp2rs_gpx.py

@@ -1621,8 +1621,13 @@ def convert_file(
                                     # the shape over time (§6.2.1). value/divisor
                                     # = semitones (scale auto-detected per file).
                                     if 'Bended' in _tp:
+                                        # Use the beat duration `dur`, not
+                                        # `rn.sustain` (zeroed for notes <= 0.2s),
+                                        # so short bends keep their bnv curve —
+                                        # matching the GP5 path, which maps over
+                                        # the raw note duration.
                                         _peak, _intent, _curve = _gpx_bend_shape(
-                                            _tp, _bend_divisor, rn.sustain)
+                                            _tp, _bend_divisor, dur)
                                         if _peak > 0:
                                             rn.bend = _peak
                                             rn.bend_intent = _intent