feat(sloppak): load multiple drum parts (feedpak 1.17.0 drums-as-arrangements) (#1020)

* feat(sloppak): load multiple drum parts (feedpak 1.17.0 drums-as-arrangements)

A song can now ship SEVERAL drum charts (a second drummer, an aux-percussion
layer). The Arrangement Editor already writes them per the feedpak 1.17.0 FEP
(feedpak-spec#63): the primary stays the song-level `drum_tab:` key (what this
app has always played), and each part rides the manifest as a `type: drums`
arrangement entry carrying a per-arrangement `drum_tab` file pointer and NO
note `file` — an entry this loader's file/notation gate already skips, which
is exactly why old builds are unaffected by such packs.

lib/sloppak.py:
- The arrangements loop collects drum-part pointer entries instead of merely
  skipping them — but still NEVER turns one into a fretted Arrangement. That
  skip is the grading invariant (an empty drum chart must not reach the
  fretted pipeline / note-detection grading) and is now pinned by test.
- New `LoadedSloppak.drum_parts`: [{id, name, drum_tab}], primary FIRST. The
  entry aliasing the song-level file contributes its id/name but is never
  loaded twice (the primary's payload IS `loaded.drum_tab`, same object).
  Legacy single-drum packs read as a one-part list; a pointer-only pack (a
  writer omitted the alias) promotes its first part so has_drum_tab, the
  default stream, and the drum-only placeholder keep working.
- The song-level drum_tab loading block is extracted verbatim into
  `_load_drum_tab_file()` and shared by both paths, so every part gets the
  same permissive posture: missing file → that part silently absent;
  traversal / parse / validation failure → that part skipped with a warning,
  never an aborted load. (The 9 pinned drumtab-load tests pass unchanged.)

lib/routers/ws_highway.py:
- `song_info` gains `drum_parts` (names only; always a list, empty without
  drums) so a part picker can bind unconditionally.
- `?drum_part=<id>` on the WS URL selects which part's tab streams as the
  `drum_tab`/`drum_hits` messages; the default and any unknown id fall back
  to the primary — byte-identical legacy behavior. The `drum_tab` message
  carries `part_id` only when a parts list exists, keeping the legacy frame
  unchanged.

Tests: tests/test_sloppak_drum_parts.py (9) — the grading invariant +
parallel-ids pin, primary-first resolution with alias identity, legacy
one-part list, pointer-only promotion, per-part failure isolation (bad JSON,
path traversal, duplicate rels), and the drum-only placeholder with pointer
entries. Full suite: the only failures are 9 machine-environmental tests
(installed desktop plugins under LOCALAPPDATA, CRLF/path-shape assertions)
that fail identically on an untouched origin/main checkout on this box.
tools/check_spec_conformance.py passes against the spec's current HEAD
(`drum_tab` and `type` are declared keys); the semantics of the
per-arrangement placement land in feedpak-spec#63 — this PR should merge
after it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xGPjDBF8NTwTK7VQvizix
Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

* Fix drum-part review findings

* Normalize drum part pointer identities

* fix(sloppak): enforce drums grading invariant + green the suite

- Gate the drum-pointer skip on type FIRST: a type:drums/drum entry never becomes
  a fretted Arrangement even if it carries a note file/notation (with drum_tab it
  is collected as a drum part, without it dropped+warned). Closes the spec
  §5.2/§7.5 MUST-NOT hole (a malformed drums+file entry was being fretted-graded).
- Make test_drum_pointer_with_wrong_type_logs_warning robust (attach handler to the
  feedBack logger + set WARNING, restore in finally) and fix the root-cause level
  leak in test_tuning_provider_isolation.py (finally restored the handler but not
  the level, leaking ERROR onto the feedBack tree and turning the suite red under
  full ordering).
- Restore the chart-transform CHANGELOG bullet (#952) the drum entry had truncated.

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

---------

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

CHANGELOG.md

@@ -8,6 +8,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Multiple drum parts (feedpak 1.17.0 "drums as arrangements").** The sloppak
+  loader now reads `type: drums` arrangement entries carrying per-arrangement
+  `drum_tab` file pointers — a song can ship several drum charts (a second
+  drummer, an aux-percussion layer). Parts surface as `LoadedSloppak.drum_parts`
+  (primary first; the entry aliasing the song-level `drum_tab:` key is the
+  primary and is never loaded twice), the highway WS `song_info` gains a
+  `drum_parts` name list, and `?drum_part=<id>` on the WS URL selects which
+  part's tab streams (`drum_tab` messages carry `part_id` when multiple parts
+  exist; unknown ids fall back to the primary). Pointer entries are **never**
+  loaded as fretted arrangements — the loader's file/notation gate keeps a drum
+  part out of the fretted pipeline (and out of note-detection grading), pinned
+  by test. Legacy single-drum packs read exactly as before, as a one-part list.
 - **`chart-transform` capability domain (#952)** — plugins can now remap the
   chart before rendering and scoring through a core-owned provider
   coordinator. Synchronous transforms run after difficulty filtering; host