Partner commanders: Partner, Partner—<variant>, Partner with, Background, Doctor

A Commander/Brawl deck can now have two commanders when their abilities
allow it. Partner abilities are parsed from oracle text (Scryfall's keyword
list files every "Partner—Survivors"-style variant under plain "Partner"),
and a second search box offers only cards that can pair with the commander.
The legality check accepts a valid pair and checks color identity against
both; suggestions and decklist import/export handle both commanders.

Closes #4

src/api.ts

@@ -29,6 +29,13 @@ export interface LibraryCard extends CardRef {
   location: string;
 }
 
+/** A partner-style ability, letting a card share the command zone with a
+ *  second commander. Mirrors `mtgc_core::partner::Partner`. */
+export type Partner =
+  | { kind: "partner" | "chooses_background" | "background" | "doctors_companion" | "doctor" }
+  /** "Partner—Survivors" and the like (`variant`); "Partner with <name>" (`with`). */
+  | { kind: "variant" | "with"; name: string };
+
 export interface DeckEntry extends CardRef {
   quantity: number;
   is_commander: boolean;
@@ -38,6 +45,7 @@ export interface DeckEntry extends CardRef {
   quantity_owned: number;
   /** The printing whose art this slot shows; `image_uri` already reflects it. */
   preferred_scryfall_id: string | null;
+  partner: Partner | null;
 }
 
 /** One printing of a card, for the printing picker and copy labels. */
@@ -487,6 +495,12 @@ export const api = {
     invoke<void>("set_deck_format", { deckId, format }),
   setDeckCommander: (deckId: number, oracleId: string | null) =>
     invoke<void>("set_deck_commander", { deckId, oracleId }),
+  /** One commander, or a partner pair; the first is the deck's primary. */
+  setDeckCommanders: (deckId: number, oracleIds: string[]) =>
+    invoke<void>("set_deck_commanders", { deckId, oracleIds }),
+  /** Name search narrowed to cards that could partner with the commander. */
+  searchPartners: (commanderOracleId: string, query: string, limit: number) =>
+    invoke<CardRef[]>("search_partners", { commanderOracleId, query, limit }),
   setDeckCard: (
     deckId: number,
     oracleId: string,