feat(core): per-note bend shape (bt + bnv) — wire + GP import (#531)

Implements feedpak spec §6.2.1 (feedpak 1.4.0) per-note bend shape on the
core side:

- `bn` stays the bend's peak magnitude in semitones (unchanged).
- `bt` — bend intent (0 up, 1 release, 2 pre-bend, 3 pre-bend-release,
  4 round-trip), default 0, default-omitted on the wire.
- `bnv` — time-stamped bend curve [{t: seconds-from-onset, v: semitones}],
  authoritative when present; default-omitted. Older readers ignore both.

Wire (lib/song.py): Note.bend_intent/bend_values; note_to_wire emits bt/bnv
only when set; note_from_wire reads them via _sanitize_bend_curve (drops
malformed entries, empty -> None never []). _parse_note reads them from the
GP-import XML (bendIntent attr + bendValues JSON) so GP curves survive
import -> XML -> wire -> highway.

GP5 (lib/gp2rs.py): _gp_bend_shape maps pyguitarpro BendPoints to a bnv
curve — semitones = value/2.0 (consistent with the existing scalar bn),
t = position/12 * duration — and _bend_intent_from_values derives bt from
the shape. Emitted for <note> and <chordNote> via the shared _build_xml.

GP8 (lib/gp2rs_gpx.py): _gpx_bend_shape builds a 3-point curve from the
GPIF origin/middle/destination value+offset Properties (value/divisor
semitones, offset/100 * sustain seconds), reusing the shared _build_xml.
GPIF offset Property names should be confirmed against a real GP8 export.

Tests cover wire round-trip + default-omit + sanitization, GP5 unit/time
mapping + intent classification end-to-end through the XML, and the GP8
curve builder.

Part of got-feedback/feedback#334

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

tests/test_gp2rs_gpx.py

@@ -31,6 +31,7 @@ from gp2rs_gpx import (
     _collect_tone_events,
     _inject_tones,
     _resolve_pending_slides,
+    _gpx_bend_shape,
 )
 from gp2rs import RsNote
 
@@ -55,6 +56,50 @@ def test_safe_filename_stem(name, expected):
     assert ".." not in out
 
 
+# ── _gpx_bend_shape (bn / bt / bnv, §6.2.1) ─────────────────────────────────
+
+def _bend_props(**vals):
+    """Build a GPIF property map {name: <Property> element} for the given
+    bend Float values, e.g. _bend_props(BendOriginValue=0, BendMiddleValue=100)."""
+    tp = {}
+    for name, num in vals.items():
+        tp[name] = ET.fromstring(
+            f'<Property name="{name}"><Float>{num}</Float></Property>')
+    return tp
+
+
+def test_gpx_bend_shape_round_trip_curve():
+    """origin/middle/destination value+offset → 3-point bnv; value/divisor=semis."""
+    tp = _bend_props(
+        BendOriginValue=0, BendOriginOffset=0,
+        BendMiddleValue=100, BendMiddleOffset1=50,   # 100/50 = 2 semitones
+        BendDestinationValue=0, BendDestinationOffset=100,
+    )
+    peak, intent, curve = _gpx_bend_shape(tp, divisor=50.0, sustain=1.0)
+    assert peak == 2.0
+    assert intent == 4   # round-trip (up then back down)
+    assert curve == [
+        {"t": 0.0, "v": 0.0}, {"t": 0.5, "v": 2.0}, {"t": 1.0, "v": 0.0}]
+
+
+def test_gpx_bend_shape_falls_back_to_even_spacing_without_offsets():
+    tp = _bend_props(BendOriginValue=0, BendDestinationValue=100)  # no offsets
+    peak, intent, curve = _gpx_bend_shape(tp, divisor=50.0, sustain=1.0)
+    assert peak == 2.0
+    assert intent == 0   # plain up
+    # origin defaults to 0%, destination to 100%.
+    assert curve == [{"t": 0.0, "v": 0.0}, {"t": 1.0, "v": 2.0}]
+
+
+def test_gpx_bend_shape_no_props_and_zero_length():
+    assert _gpx_bend_shape({}, divisor=50.0, sustain=1.0) == (0.0, 0, None)
+    # Peak + intent still derived for a zero-length note, but no curve.
+    peak, intent, curve = _gpx_bend_shape(
+        _bend_props(BendOriginValue=0, BendDestinationValue=100),
+        divisor=50.0, sustain=0.0)
+    assert peak == 2.0 and intent == 0 and curve is None
+
+
 # ── _decompress_bcfz / _parse_bcfs input guards ─────────────────────────────
 
 def test_decompress_bcfz_rejects_bad_magic():