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>

tests/test_tones.py

@@ -6,16 +6,17 @@ from tones import sloppak_tone_changes
 # ── sloppak_tone_changes (highway payload builder) ───────────────────────────
 
 def test_sloppak_tone_changes_sorts_and_returns_base():
-    base, changes = sloppak_tone_changes({
+    base, base_rig, changes = sloppak_tone_changes({
         "base": "Clean",
         "changes": [{"t": 12.5, "name": "Drive"}, {"t": 3.0, "name": "Clean"}],
     })
     assert base == "Clean"
+    assert base_rig == ""
     assert changes == [{"t": 3.0, "name": "Clean"}, {"t": 12.5, "name": "Drive"}]
 
 
 def test_sloppak_tone_changes_skips_malformed_markers():
-    _, changes = sloppak_tone_changes({
+    _, _, changes = sloppak_tone_changes({
         "changes": [
             {"t": "nan", "name": "BadStr"},
             {"t": float("inf"), "name": "Inf"},
@@ -29,18 +30,66 @@ def test_sloppak_tone_changes_skips_malformed_markers():
 
 
 def test_sloppak_tone_changes_handles_none_and_bad_base():
-    assert sloppak_tone_changes(None) == ("", [])
-    base, changes = sloppak_tone_changes({"base": 123, "changes": []})
-    assert base == "" and changes == []
+    assert sloppak_tone_changes(None) == ("", "", [])
+    base, base_rig, changes = sloppak_tone_changes({"base": 123, "changes": []})
+    assert base == "" and base_rig == "" and changes == []
 
 
 def test_sloppak_tone_changes_non_dict_input():
     """A truthy non-dict payload must not crash."""
-    assert sloppak_tone_changes(["not", "a", "dict"]) == ("", [])
-    assert sloppak_tone_changes("nope") == ("", [])
+    assert sloppak_tone_changes(["not", "a", "dict"]) == ("", "", [])
+    assert sloppak_tone_changes("nope") == ("", "", [])
 
 
 def test_sloppak_tone_changes_non_list_changes():
     """A truthy non-list `changes` value must not raise on iteration."""
-    base, changes = sloppak_tone_changes({"base": "Clean", "changes": 1})
+    base, _, changes = sloppak_tone_changes({"base": "Clean", "changes": 1})
     assert base == "Clean" and changes == []
+
+
+# ── rig bindings (feedpak-spec 1.18.0 §6.9) ──────────────────────────────────
+
+def test_sloppak_tone_changes_carries_rig_bindings():
+    """`base_rig` and per-change `rig` reach the wire — the binding a chart
+    declares is what core must hand the consumer that voices the part."""
+    base, base_rig, changes = sloppak_tone_changes({
+        "base": "Clean Rhythm",
+        "base_rig": "clean-rhythm",
+        "changes": [
+            {"t": 12.5, "name": "Lead Drive", "rig": "lead-drive"},
+            {"t": 48.0, "name": "Clean Rhythm", "rig": "clean-rhythm"},
+        ],
+    })
+    assert base == "Clean Rhythm"
+    assert base_rig == "clean-rhythm"
+    assert changes == [
+        {"t": 12.5, "name": "Lead Drive", "rig": "lead-drive"},
+        {"t": 48.0, "name": "Clean Rhythm", "rig": "clean-rhythm"},
+    ]
+
+
+def test_sloppak_tone_changes_omits_unusable_rig_ids():
+    """A non-string or blank `rig` is dropped rather than forwarded, so a
+    consumer can treat presence of the key as "this change binds a rig"."""
+    _, base_rig, changes = sloppak_tone_changes({
+        "base_rig": "   ",
+        "changes": [
+            {"t": 1.0, "name": "A", "rig": 7},
+            {"t": 2.0, "name": "B", "rig": ""},
+            {"t": 3.0, "name": "C", "rig": None},
+            {"t": 4.0, "name": "D", "rig": "  padded-id  "},
+        ],
+    })
+    assert base_rig == ""
+    assert changes == [
+        {"t": 1.0, "name": "A"},
+        {"t": 2.0, "name": "B"},
+        {"t": 3.0, "name": "C"},
+        {"t": 4.0, "name": "D", "rig": "padded-id"},
+    ]
+
+
+def test_sloppak_tone_changes_non_string_base_rig():
+    """A non-string `base_rig` must not crash or leak a non-id onto the wire."""
+    _, base_rig, _ = sloppak_tone_changes({"base": "Clean", "base_rig": 42})
+    assert base_rig == ""