feat(types): BigInt + Decimal exact economy numbers (#52)

Types phase 2 — the "money problem". A splice-on-demand bignum engine
(runtime/native/bignum.ludic, self-contained, only intrinsics) exposed as
two namespaces:

- BigInt.* — arbitrary-precision integer (sign-magnitude, base-1e9 limbs):
  from/parse, add/sub/mul/pow, div/mod (by int), cmp/eq/is_zero, to_int, str.
  For idle counters and exact huge currencies that overflow a 32/64-bit int.
- Decimal.* — exact base-10 fixed point (BigInt mantissa + decimal scale):
  from/parse, exact add/sub/mul, cmp/eq, scale/rescale (truncate), str.
  So 0.10 + 0.20 is exactly 0.30 — no binary rounding.

Both exact => deterministic; no f32/f64. Wired: parser splice trigger
(g_uses_bignum), emit_call dispatch, reseeded seed, a self-asserting example
(examples/library/bignum.ludic + feat_case), and per-symbol docs + inventory.
All suites green incl. golden renders byte-identical and the bootstrap fixpoint.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/inventory.json

@@ -383,6 +383,35 @@
     "rect-contains",
     "rect-intersects"
   ],
+  "bigint": [
+    "bigint-from",
+    "bigint-parse",
+    "bigint-add",
+    "bigint-sub",
+    "bigint-mul",
+    "bigint-neg",
+    "bigint-pow",
+    "bigint-div",
+    "bigint-mod",
+    "bigint-cmp",
+    "bigint-eq",
+    "bigint-is_zero",
+    "bigint-to_int",
+    "bigint-str"
+  ],
+  "decimal": [
+    "decimal-from",
+    "decimal-parse",
+    "decimal-add",
+    "decimal-sub",
+    "decimal-mul",
+    "decimal-neg",
+    "decimal-cmp",
+    "decimal-eq",
+    "decimal-scale",
+    "decimal-rescale",
+    "decimal-str"
+  ],
   "duration": [
     "duration-seconds",
     "duration-minutes",