harden: a holders_count that is not plain digits is unknown, not read in part (closes #251)

parseHoldersCount used parseInt, which reads "1,000" as 1, "0x10" as 0 and
"1e3" as 1: a reported low count, which hides the token in the transaction
history and the send-screen token selector. It now accepts only a whole
number of zero or more, or a string of digits alone, and returns null for
anything else. The balance list's holders !== null check did nothing, since
null >= 1000 is already false, and is dropped. README.md and docs/README.md
say how each filter treats an unknown count and that the token screen then
leaves out its Holders row; README.md lists src/shared/holders.js.

Model: opus-5-5

docs/README.md

@@ -333,7 +333,13 @@ it is hidden from your transaction history and from the send token list.
 from transaction history and the send token list, and are left out of your
 balances unless they are on the bundled known-token list or you added them
 yourself. Legitimate tokens have substantial holder counts; scam tokens deployed
-for address poisoning typically have zero.
+for address poisoning typically have zero. When the explorer reports no holder
+count for a token, or reports something other than a whole number in plain
+digits (such as "1,000"), the count is unknown. An unknown count does not hide a
+token from your transaction history or the send token list, and it does not get
+a token into your balances either: such a token is listed only if it is on the
+bundled known-token list or you added it yourself. The screen you reach by
+clicking a token balance shows a "Holders:" line only when the count is known.
 
 **Fraud contract blocklist.** When AutistMask detects a fraudulent transfer, it
 adds the contract address to a local blocklist. Future transactions from that