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, no larger than
Number.MAX_SAFE_INTEGER, 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

src/shared/holders.js

@@ -9,13 +9,22 @@
 
 const LOW_HOLDER_THRESHOLD = 1000;
 
-// Parse an explorer-supplied holders_count into a number, or null when the
-// explorer did not report one.  Anything unparseable is unknown too: a count
-// we cannot read is not a count of zero.
+// Parse an explorer-supplied holders_count into a number, or null when it is
+// not one.  Only a whole number of zero or more, or a string made of nothing
+// but the digits 0-9, is a count.  Anything else is null, never read in part:
+// "1,000", "0x10" and "1e3" are unknown, not 1, 0 and 1, because a count we
+// cannot read is not a low count.  A count above Number.MAX_SAFE_INTEGER is
+// null too: a number cannot hold it exactly, so it would come back rounded,
+// or as Infinity.
 function parseHoldersCount(raw) {
-    if (raw === null || raw === undefined || raw === "") return null;
-    const n = parseInt(raw, 10);
-    return Number.isFinite(n) ? n : null;
+    if (typeof raw === "number") {
+        return Number.isSafeInteger(raw) && raw >= 0 ? raw : null;
+    }
+    if (typeof raw === "string" && /^[0-9]+$/.test(raw)) {
+        const count = Number(raw);
+        return Number.isSafeInteger(count) ? count : null;
+    }
+    return null;
 }
 
 // True only for a token the explorer reported as having fewer holders than