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.

TODO.md

@@ -45,6 +45,20 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: `KNOWN_SYMBOLS` now maps a symbol to the set of contract addresses
+  that bear it, not to one of them. A ticker is not unique: seven of the 512
+  bundled tokens — `FRAX`, `REUSD`, `TON`, `EURE`, `MSUSD`, `MUSD` and `JPYC` —
+  share a symbol with another bundled entry at a different real contract, and
+  the table, built from the list first-wins, kept only the earlier one. The
+  other seven were judged spoofs of their own symbol at their own address and
+  hidden from the balance list, the history and the send selector, so a holder
+  could not spend them. Both contracts of each pair come from the same CoinGecko
+  fetch of 2026-02-27, so neither was stale and neither was dropped.
+  `isSpoofedSymbol()` asks set membership instead of equality, which does not
+  loosen the rule — a contract outside the set is still a spoof — and a test now
+  walks `TOKENS` asserting no bundled token is filtered at its own address,
+  which is the walk the suite lacked
+  ([#276](https://git.eeqj.de/sneak/AutistMask/issues/276)).
 - 2026-08-12: The dApp approval round trips are driven end to end in the
   browser. A test page served by the harness speaks EIP-1193 to the real inpage
   provider through the real content script, background worker and approval popup