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

KNOWN_SYMBOLS maps a symbol to the set of contract addresses that bear
it, instead of to one of them.

A ticker is not unique, and the bundled list proves it: seven of its 512
tokens -- FRAX, REUSD, TON, EURE, MSUSD, MUSD and JPYC -- share a symbol
with another bundled entry at a different real contract.  The table is
built from that list first-wins, so it kept the earlier entry of each
pair and the later one was judged a spoof of its own symbol at its own
address.  A user holding any of the seven saw it filtered out of the
balance list, the transaction history and the send token selector, and
so could not spend it through the UI.

Both contracts of every pair come from the same source fetch (CoinGecko,
2026-02-27, decimals verified on-chain), so neither is stale relative to
the other and there is nothing to prefer between them.  The fix is
therefore in the shape of the table rather than in its contents: no
address was picked and none was dropped.  isSpoofedSymbol() asks set
membership where it asked equality, which does not loosen the rule --
every address in a set is one the wallet ships as a real token, and a
contract outside the set is still a spoof.  The native-asset entry stays
null and still means no contract may bear the symbol.

The suite walked KNOWN_SYMBOLS, which is derived from TOKENS, so it
could only assert that the table agreed with itself.  It now also walks
TOKENS asserting that no bundled token is filtered at its own address --
the walk that would have caught this -- pins both contracts of each of
the seven by address, asserts a third contract bearing a shared ticker
is still filtered, and asserts every address the table vouches for is a
bundled token reporting that symbol.

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", () => {