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>

changes/types-bignum-decimal.md

@@ -0,0 +1,13 @@
+bump: minor
+type: feat
+**Exact numbers for game economies — `BigInt` + `Decimal` (#52, types phase 2).**
+The thing a tycoon or idle game must never round wrong. `BigInt.*` is an
+arbitrary-precision integer (sign-magnitude, base-1e9 limbs) with `from`/`parse`,
+`add`/`sub`/`mul`/`pow`, integer `div`/`mod`, `cmp`/`eq`/`is_zero`, `to_int` and
+`str` — for idle counters and exact huge currencies that overflow a 32/64-bit
+int. `Decimal.*` is an exact base-10 fixed-point number (a `BigInt` mantissa
+plus a decimal scale) with `from`/`parse`, exact `add`/`sub`/`mul`, `cmp`/`eq`,
+`scale`/`rescale` and `str` — so money like `0.10 + 0.20` is exactly `0.30`, no
+binary rounding. Both are exact and therefore deterministic; no `f32`/`f64`. The
+engine (runtime/native/bignum.ludic) is spliced in on demand, so it costs
+nothing when unused and works in plain tools as well as games.