fix(thressgame-coverage): F1+F4 remediation (T58 RequestChoiceModal + coin-flip kind)

Final Verification Wave found two real blockers:
1. T58 RequestChoiceModal.tsx was marked complete but did NOT exist on disk.
2. request-choice locked 6-kind enum was shipped as 5 (missing 'coin-flip').

Remediation:
- Build RequestChoiceModal.tsx with role=dialog, aria-modal=true, ESC/backdrop close, kind-specific input UI for all 6 kinds (rps / coin-flip / piece / square / column / row); 4 tests
- Add 'coin-flip' to:
  - request-choice primitive paramsSchema enum
  - PendingChoice.kind union (schema.ts + util/pending-choices.ts)
  - WS protocol ChoiceKindSchema (server/protocol.ts)
  - choice-timeout.firstDefaultForKind (defaults to 'heads')
  - broadcast.isValidChoiceValue (accepts 'heads' | 'tails')
- AutoChoiceResolver: deterministic alternating heads/tails for coin-flip

T68 e2e tests remain .skip()'d pending UI integration (modal-into-GameView wiring + activate-descriptor UI) — a follow-up task. The sentinel test asserts the gap exists so when integration lands, skips lift in the documented order.

Tests: 2740 -> 2744 (+4). bun run check exit 0.

packages/server/src/broadcast.ts

@@ -526,6 +526,8 @@ function isValidChoiceValue(
         value >= 0 &&
         value <= 7
       );
+    case "coin-flip":
+      return value === "heads" || value === "tails";
   }
 }
 

packages/server/src/choice-timeout.ts

@@ -74,7 +74,7 @@ import {
  */
 export function firstDefaultForKind(
   kind: PendingChoice["kind"],
-): "rock" | number {
+): "rock" | number | "heads" {
   switch (kind) {
     case "rps":
       return "rock";
@@ -84,6 +84,8 @@ export function firstDefaultForKind(
     case "column":
     case "row":
       return 0;
+    case "coin-flip":
+      return "heads";
   }
 }
 

packages/server/src/protocol.test.ts

@@ -1252,7 +1252,7 @@ describe("T43 — RequestChoiceSchema", () => {
   it("rejects an unknown choiceKind", () => {
     const r = RequestChoiceSchema.safeParse({
       ...validRequestChoice,
-      choiceKind: "coin-flip",
+      choiceKind: "invalid-kind-that-will-never-exist",
     });
     expect(r.success).toBe(false);
   });

packages/server/src/protocol.ts

@@ -1077,6 +1077,7 @@ export const ChoiceKindSchema = z.enum([
   "square",
   "column",
   "row",
+  "coin-flip",
 ]);
 export type ChoiceKind = z.infer<typeof ChoiceKindSchema>;