fix: a shared ticker no longer hides one of its two real tokens (closes #276)

Seven bundled tokens were filtered as spoofs at their own address, so a user
holding FRAX, TON, REUSD, EURE, MSUSD, MUSD or JPYC could not see or spend the
one the wallet happened not to pick.

The known-symbol table is derived from the bundled token list, first-wins in
market-cap order, so a symbol that appears twice silently condemned its second
contract. Both are real tokens from the same fetch and neither is stale --
three pairs are one issuer's old and new contract, four are unrelated issuers
sharing a ticker. Picking a winner would have been guessing, and dropping the
ambiguous symbols would have ended spoof filtering for those tickers entirely.

The table now maps a symbol to the set of addresses that legitimately bear it.
A contract outside the set is still a spoof, so the check is not weakened: a
third contract bearing any of the seven shared tickers is refused, and that is
tested. The filter decides what is fake, not what is worth holding, so a legacy
contract stays in the set -- it still holds real balances.

A test walks the whole bundled list asserting no token is filtered at its own
address, which is the guard whose absence let this ship.

tests/transactions.test.js

@@ -207,8 +207,8 @@ describe("token list assumptions the fixtures rely on", () => {
     });
 
     test("USDC and WETH map to their genuine lowercased contracts", () => {
-        expect(KNOWN_SYMBOLS.get("USDC")).toBe(USDC_CONTRACT);
-        expect(KNOWN_SYMBOLS.get("WETH")).toBe(WETH_CONTRACT);
+        expect([...KNOWN_SYMBOLS.get("USDC")]).toEqual([USDC_CONTRACT]);
+        expect([...KNOWN_SYMBOLS.get("WETH")]).toEqual([WETH_CONTRACT]);
     });
 
     test("the spam fixture symbol is not in the known token list", () => {