schemagen: fix quadratic leading-zero strip in integer fallback

Replace the O(k^2) loop that erased leading zeros one byte at a time
from the front of a std::string with a single linear scan and one
erase(0, n) call.

Also adds regression tests for issue #34:
- correctness cases for numbers with leading fractional zeros
- a static check that the generated code no longer contains the
  quadratic pattern
- a large-input case (100k leading zeros) that reproduces the
  vulnerable shape

Closes #34

contrib/schemagen/weaseljson_schemagen.py

@@ -1000,8 +1000,10 @@ private:
     }}
     int64_t finalExp = exp - fracDigits + trim;
     if (finalExp < 0) return false;
-    while (!digits.empty() && digits.front() == '0') digits.erase(digits.begin());
-    if (digits.empty()) {{ out = 0; return true; }}
+    size_t leadingZeros = 0;
+    while (leadingZeros < digits.size() && digits[leadingZeros] == '0') ++leadingZeros;
+    if (leadingZeros == digits.size()) {{ out = 0; return true; }}
+    if (leadingZeros > 0) digits.erase(0, leadingZeros);
 
     constexpr uint64_t kMaxNeg = 9223372036854775808ULL;
     constexpr uint64_t kMaxPos = 9223372036854775807ULL;