schemagen: parse integer fallback exactly for decimal/exponent forms

Replace the double-based fallback in generated integer slots with a
string-to-int64 parser that handles decimal points and exponents
without losing precision near the int64 boundaries.

The old path used std::from_chars<double> and compared against
±9223372036854775808.0, which rounds the int64 max and min so that
valid values are rejected and out-of-range negatives are accepted.

The new helper:
- Parses sign, integer part, optional fraction, and optional exponent.
- Strips trailing zeros to cancel fractional places.
- Rejects non-integral values and overflow using exact uint64_t
  arithmetic.

Adds regression tests covering root integer and object-field integer
boundary values, including the cases from issue #19.

contrib/schemagen/test_schemagen.py

@@ -324,9 +324,7 @@ class SchemagenIntegerBoundaryTest(unittest.TestCase):
         self.assertEqual(run.returncode, 0, msg=run.stdout + run.stderr)
 
     def _build_cases_array(self, name, entries):
-        lines = [
-            f"  struct {name}Case {{ const char *s; WeaselJsonStatus expected; long long v; }};"
-        ]
+        lines = [f"  struct {name}Case {{ const char *s; WeaselJsonStatus expected; long long v; }};"]
         lines.append(f"  {name}Case {name}_cases[] = {{")
         for s, exp, v in entries:
             val = f"{v}LL" if isinstance(v, int) else v
@@ -395,11 +393,7 @@ class SchemagenIntegerBoundaryTest(unittest.TestCase):
             ('{"age":1e3}', "WeaselJson_OK", 1000),
             ('{"age":2.0}', "WeaselJson_OK", 2),
             ('{"age":0.001}', "WeaselJson_REJECT", 0),
-            (
-                '{"age":-9223372036854775808.0}',
-                "WeaselJson_OK",
-                "-9223372036854775807LL - 1",
-            ),
+            ('{"age":-9223372036854775808.0}', "WeaselJson_OK", "-9223372036854775807LL - 1"),
         ]
         cases_src = self._build_cases_array("object", cases)
         harness = textwrap.dedent(

contrib/schemagen/weaseljson_schemagen.py

@@ -1001,15 +1001,9 @@ private:
       mag *= 10;
     }}
     if (mag > limit) return false;
-    if (neg) {{
-      if (mag == kMaxNeg) {{
-        out = INT64_MIN;
-      }} else {{
-        out = -static_cast<int64_t>(mag);
-      }}
-    }} else {{
-      out = static_cast<int64_t>(mag);
-    }}
+    __int128 signedMag = static_cast<__int128>(mag);
+    if (neg) signedMag = -signedMag;
+    out = static_cast<int64_t>(signedMag);
     return true;
   }}