feat(sloppak): core reader for source rigs (feedpak 1.18.0) (#1040)

* Carry rig bindings through the sloppak tone payload

`sloppak_tone_changes` emitted `{t, name}` only, so a chart's declared
sound never reached the client: `base_rig` was never read and each
change's `rig` was dropped at the wire boundary. Both survive load
intact (`Arrangement.tones` is an opaque passthrough) — the strip
happened here, at the last step before send.

That left the rig model (feedpak-spec 1.18.0 §6.9/§7.9) unreachable
from core: a pack could declare which rig voices a part, and nothing
downstream could ever see it. First step of the core reader for source
rigs; the rig library itself and the manifest precedence cascade follow.

Return `(base, base_rig, changes)` and keep `rig` on each change. Both
ids are validated as non-blank strings and stripped — anything else is
dropped rather than forwarded, so presence of the key means the change
binds a rig. Resolution against `rigs.json` deliberately does NOT happen
here: this builder preserves the declared binding, while realization
selection and the `intent.gm` fallback belong to whatever voices the
part.

On the wire `base_rig` is omitted entirely when empty, so packs that
bind no rig produce the byte-identical `tone_changes` message they
always did.

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

* Load the pack's rig library from the manifest

feedpak 1.18.0 lets a chart declare what a MIDI part should sound like
by binding a rig id, but core had nothing to bind to: `rigs`, `base_rig`
and `drum_tones` appeared nowhere in lib/, server.py or static/. The
preceding commit carries the reference onto the wire; this adds the
library it references.

Read the manifest `rigs:` key into a new `LoadedSloppak.rigs`, alongside
the other side-files rather than on Song — every side-file (drum_tab,
song_timeline, keys, notation) hangs off the load result, and rigs is
pack-level, not per-arrangement. Same permissive posture as its
neighbours: missing, unreadable, malformed or traversing disables rigs
with a warning and never fails the pack, which §7.9 requires outright.

Rig objects pass through VERBATIM. §7.9 obliges a Reader to preserve
unknown role/engine/kind values and `ext` namespaces, so validating
block structure here would be wrong as well as premature — realization
selection and the `intent.gm` floor belong to whatever voices the part.
The only entries dropped are ones unreachable by construction: a rig is
addressable solely by `id`, so a non-dict entry or one without a usable
string id can never be referenced. Ids are stripped to match the
reference side, and a duplicate id resolves first-wins with a warning,
since ambiguity there would surface as the wrong sound rather than an
error.

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

* Resolve which tones block binds a part

feedpak 1.18.0 lets a sound binding arrive from three places, and core
honoured none of them: the manifest arrangement entry, the arrangement
JSON, and the top-level drum_tones. Reading them needs a precedence
rule, because two of the three can be present at once.

Arrangement entries: the entry's `tones` replaces the arrangement JSON's
WHOLESALE (spec 5.2), unlike name/tuning/capo/centOffset beside it,
which override field by field. A merge would produce a sound nobody
authored -- one source's base under the other's changes -- which is
worse than either block alone. This is also what makes a notation-only
keys entry bindable at all, since it has no arrangement JSON to carry
tones in the first place.

Drums: the top-level drum_tones binds the song-level primary part, and
a `type: drums` entry's own tones takes precedence, with a Reader
forbidden from applying both to the same part (5.1). That is the same
shape as the drum_tab alias rule, so it lives inside
_resolve_drum_parts next to it rather than beside it -- one precedence
resolver, not two that drift. drum_tones is the PRIMARY's fallback
only: a second drummer with no binding gets None, never the primary's
kit.

An empty `tones: {}` reads as absent rather than as an override to
silence, matching how arrangement_from_wire already normalizes the
in-JSON empty dict, so a stray empty object cannot quietly unbind a
part.

Spec-conformance gate passes with drum_tones added to the keys core
reads (22 of the spec's 32, all declared).

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

* Document the rig bindings on the tone_changes wire message

CHANGELOG entry for the core rig reader, plus the WS protocol table in
CLAUDE.md, which described `tone_changes` as carrying only base + name.

While in that row: its time key was documented as `time`, but every
producer emits `t` — both the sloppak builder and the legacy XML path.
The 3D highway already carries a comment warning readers about exactly
this discrepancy. Corrected here rather than left sitting next to the
newly-added keys, where a reader would reasonably assume both were
equally reliable.

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

---------

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

CHANGELOG.md

@@ -8,6 +8,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Core reader for source rigs (feedpak 1.18.0).** A pack can declare what a
+  MIDI part should sound like by binding a rig; core now reads that binding and
+  hands it to the client instead of dropping it. Three parts: the
+  `tone_changes` WS message carries the pack's rig bindings (`base_rig`, and
+  `rig` per change) alongside the tone names it already sent; the manifest
+  `rigs:` key loads the pack's rig library (`rigs.json`, spec §7.9) verbatim;
+  and the binding precedence is resolved per spec §5.1/§5.2 — a manifest
+  arrangement entry's `tones` replaces the arrangement JSON's **wholesale**
+  (no field-level merge), while top-level `drum_tones` binds the primary drum
+  part as the fallback a `type: drums` entry's own `tones` outranks. Core
+  deliberately stops there: it does not select a realization or apply the
+  `intent.gm` floor, which belong to whatever actually voices the part. Packs
+  that bind no rig produce a byte-identical `tone_changes` payload, so existing
+  consumers are unaffected.
 - **Opt-in career venue packs (#122)** — higher-tier venue crowd media
   (`club`, `arena`) is no longer bundled; the app downloads each pack on demand
   from its release when you reach the venue (sha256-verified), keeping the