fix(song): make bass detection instrument-type-aware, not name-only (#1019)

Editor now authors an arrangement's instrument as first-class data (a manifest
'type' field). Core dropped it: the sloppak loader never read 'type', and
'is this a bass?' was defined three different ways across call sites (name-only
in note_pitch_midi and the highway scale-degree path; path_bass+name in bass
selection; name-only in arrangement_string_count). So an authored type=bass
chart not named 'bass' got 6-string lane counts and guitar open-string MIDI.

- Add optional Arrangement.type; sloppak load_song lifts the manifest type onto it
- Add arrangement_is_bass(arr) = type=='bass' OR path_bass OR 'bass' in name
  (None/whitespace safe), and route string count, note_pitch_midi, the highway
  scale-degree base, and bass-player selection through it
- Back-compat: no bass signal -> unchanged 6-string / guitar behavior

Companion to editor #335 (first-class instrument type). Scale degrees are
display-only and never feed a grader.

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

lib/sloppak.py

@@ -935,6 +935,11 @@ def load_song(
         # the arrangement JSON (name, tuning, capo, centOffset).
         if entry.get("name"):
             arr.name = str(entry["name"])
+        # Editor-authored instrument type (feedpak-spec §5.2 / editor PR #335).
+        # Drives arrangement_string_count's bass fallback so a bass authored on
+        # an arrangement whose NAME doesn't say "bass" still reports 4 strings.
+        if entry.get("type"):
+            arr.type = str(entry["type"]).strip().lower()
         if "tuning" in entry:
             arr.tuning = list(entry["tuning"])
         if "capo" in entry: