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.

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