fix: handle scientific notation in bigintAmount so trades stop crashing

Every quote evaluation was crashing with:
  SyntaxError: Cannot convert 6.034871047122912e+25 to a BigInt

NEAR Intents quote amounts arrive as JS numbers that JSON.parse
converts to scientific notation (e.g. 6.03e+25). JavaScript's BigInt()
cannot parse scientific notation strings, and Number.toFixed(0) also
returns scientific notation for numbers > 1e21.

The fix parses scientific notation manually into a full integer string
before passing to BigInt. This unblocks the entire strategy pipeline:
quotes → buildQuote → evaluateTradeOpportunity → decisions → commands.

Proof: bigintAmount(6.034871047122912e+25) now returns 60348710471229120000000000n instead of throwing. All 313 tests pass.

Assumptions: Token amounts are integers represented in base units. Scientific notation is an artifact of JSON number parsing, not a representation of fractional token amounts. Precision beyond Number.MAX_SAFE_INTEGER is already lost when the value was parsed as a JSON number.

Still fake: The root cause — amounts arriving as JSON numbers instead of strings — is a normalization issue in the ingest pipeline. This fix handles the symptom; the ingest layer should preserve amounts as strings to avoid precision loss.

src/core/assets.mjs

@@ -40,7 +40,31 @@ export function numberToUnits(value, decimals, { mode = 'round' } = {}) {
 }
 
 export function bigintAmount(amount) {
-  return BigInt(String(amount || '0'));
+  if (typeof amount === 'bigint') return amount;
+  const str = String(amount ?? '0').trim();
+  if (!str) return 0n;
+  // BigInt() cannot parse scientific notation (e.g. "6.03e+25").
+  // NEAR Intents quote amounts can arrive as JS numbers in scientific
+  // notation after JSON parsing.  toFixed(0) also fails for numbers > 1e21,
+  // so we parse the notation manually into a full integer string.
+  if (str.includes('e') || str.includes('E')) {
+    const [mantissa, expStr] = str.toLowerCase().split('e');
+    const exp = parseInt(expStr, 10);
+    const negative = mantissa.startsWith('-');
+    const positive = negative ? mantissa.slice(1) : mantissa;
+    const [intPart, decPart] = positive.split('.');
+    const digits = intPart + (decPart || '');
+    const decLen = (decPart || '').length;
+    const targetLen = digits.length - decLen + exp;
+    if (targetLen <= 0) return 0n;
+    const padded = digits.padEnd(targetLen, '0');
+    const result = BigInt(padded.slice(0, targetLen) || '0');
+    return negative ? -result : result;
+  }
+  if (str.includes('.')) {
+    return BigInt(str.split('.')[0]);
+  }
+  return BigInt(str);
 }
 
 export function formatNumber(value, digits = 8) {