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>

docs/language/decimal/decimal-parse.md

@@ -0,0 +1,22 @@
+---
+id: decimal-parse
+name: Decimal.parse
+category: decimal
+kind: namespace-method
+tokens: Decimal.parse
+sig: Decimal.parse(text) -> Decimal
+tip: Parse decimal text like "19.99".
+order: 1
+ns: Decimal
+member: parse
+---
+
+Parses text like <code>"19.99"</code> or <code>"0.10"</code> into an exact <code>Decimal</code>, remembering how many digits followed the point. This is how you enter a price the way binary floating point never could.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let price = Decimal.parse("19.99")
+  }
+}
+```