A test pins that a passkey belongs to its account: `findPasskey`, `removePasskey` and `notePasskeyUsed` never cross accounts #6797

src/core/passkeys.ts keeps the public half of each account's passkeys; no test calls findPasskey, removePasskey, notePasskeyUsed, passkeyIds or passkeyRoom directly.

Done when a new test file proves, with two accounts holding a passkey each:

• [ ] removePasskey with account A and B's passkey id removes nothing (B's is still listed); with A's own it removes it.
• [ ] findPasskey finds a credential by its id and returns its owner; an unknown id is undefined.
• [ ] notePasskeyUsed updates only that credential's counter/last-used, as the module documents.
• [ ] passkeyRoom says no once an account holds the most it may, as its doc comment says.

Test only; a bug it finds is fixed in the same pull.