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/_section.md

@@ -0,0 +1,7 @@
+---
+id: decimal
+title: Decimal
+order: 11
+---
+
+Exact base-10 fixed-point numbers for game economies — prices, balances and taxes on values like <code>0.10</code> that binary floating point cannot represent, so they always add up exactly. A <code>Decimal</code> is a <code>BigInt</code> mantissa with a decimal <code>scale</code> (the number of digits after the point), giving unbounded range and exact <code>add</code> / <code>sub</code> / <code>mul</code>. Build one with <code>Decimal.from</code> (an <code>int</code>) or <code>Decimal.parse</code> (text like <code>"19.99"</code>), compare with <code>cmp</code> / <code>eq</code>, change precision with <code>rescale</code> (truncates toward zero), read the current precision with <code>scale</code>, and render with <code>str</code>. Every operation is exact and deterministic. The runtime is spliced in only when a program mentions <code>Decimal.*</code> (or <code>BigInt.*</code>).