feat(thorchain): add denom field to ThorchainMsgSend (field 11)

[BitHighlander]

What

Adds optional string denom = 11; to ThorchainMsgSend.

 message ThorchainMsgSend {
     optional uint64 amount = 8 [jstype = JS_STRING];
     optional OutputAddressType address_type = 9;
     reserved 10;
+    optional string denom = 11;         // asset denom, e.g. "rune" or IBC denom
 }

Why

THORChain MsgSend now carries non-RUNE native assets — TCY, RUJIRA (RUJI),
and IBC-denominated tokens. The device needs the asset denom to display and
sign these correctly; today ThorchainMsgSend can only express RUNE.

Wire compatibility

• New optional field, tag 11 — additive, backward compatible.
• Does not touch reserved 10.
• Clients that omit denom are unaffected (RUNE-only behavior preserved).

Downstream

Consumed by keepkey-firmware (denom charset validation + display) and
python-keepkey test vectors, both staged to pin this commit once merged.

[BitHighlander]

Consolidating into a single comprehensive device-protocol PR covering the entire firmware release (not per-field).