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>

selfhost/backend/emit_call.ludic

@@ -332,6 +332,39 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
   # tile char, e.g. '#'. line/flood/a_star return []Cell slices. (Pathfinding
   # lives under Grid rather than a `Path` namespace — that name is the filesystem
   # paths library.)
+  # BigInt.* / Decimal.* -> the bignum engine (runtime/native/bignum.ludic,
+  # spliced on demand). These are ordinary Ludic functions, so the generic call
+  # path resolves them to @fn_bigint_* / @fn_decimal_* and keeps their return
+  # types (BigNum / Dec / int / bool / string).
+  if (ns == "BigInt") {
+    if (meth == "from")    { bare = "bigint_from";     push(labels, "value") }
+    if (meth == "parse")   { bare = "bigint_from_str"; push(labels, "text") }
+    if (meth == "add")     { bare = "bigint_add";      push(labels, "a"); push(labels, "b") }
+    if (meth == "sub")     { bare = "bigint_sub";      push(labels, "a"); push(labels, "b") }
+    if (meth == "mul")     { bare = "bigint_mul";      push(labels, "a"); push(labels, "b") }
+    if (meth == "neg")     { bare = "bigint_neg";      push(labels, "a") }
+    if (meth == "pow")     { bare = "bigint_pow";      push(labels, "a"); push(labels, "exp") }
+    if (meth == "div")     { bare = "bigint_div_int";  push(labels, "a"); push(labels, "d") }
+    if (meth == "mod")     { bare = "bigint_mod_int";  push(labels, "a"); push(labels, "d") }
+    if (meth == "cmp")     { bare = "bigint_cmp";      push(labels, "a"); push(labels, "b") }
+    if (meth == "eq")      { bare = "bigint_eq";       push(labels, "a"); push(labels, "b") }
+    if (meth == "is_zero") { bare = "bigint_is_zero";  push(labels, "a") }
+    if (meth == "to_int")  { bare = "bigint_to_int";   push(labels, "a") }
+    if (meth == "str")     { bare = "bigint_str";      push(labels, "a") }
+  }
+  if (ns == "Decimal") {
+    if (meth == "from")    { bare = "decimal_from";     push(labels, "value") }
+    if (meth == "parse")   { bare = "decimal_from_str"; push(labels, "text") }
+    if (meth == "add")     { bare = "decimal_add";      push(labels, "a"); push(labels, "b") }
+    if (meth == "sub")     { bare = "decimal_sub";      push(labels, "a"); push(labels, "b") }
+    if (meth == "mul")     { bare = "decimal_mul";      push(labels, "a"); push(labels, "b") }
+    if (meth == "neg")     { bare = "decimal_neg";      push(labels, "a") }
+    if (meth == "cmp")     { bare = "decimal_cmp";      push(labels, "a"); push(labels, "b") }
+    if (meth == "eq")      { bare = "decimal_eq";       push(labels, "a"); push(labels, "b") }
+    if (meth == "scale")   { bare = "decimal_scale";    push(labels, "d") }
+    if (meth == "rescale") { bare = "decimal_rescale";  push(labels, "d"); push(labels, "places") }
+    if (meth == "str")     { bare = "decimal_str";      push(labels, "d") }
+  }
   if (ns == "Grid") {
     if (meth == "line")          { bare = "grid_line";          push(labels, "x0"); push(labels, "y0"); push(labels, "x1"); push(labels, "y1") }
     if (meth == "blocked")       { bare = "grid_blocked";       push(labels, "x"); push(labels, "y"); push(labels, "wall") }

selfhost/frontend/parse.ludic

@@ -175,6 +175,7 @@ function p_postfix() -> Node {
   while true {
     if is_op(".") { pi = pi + 1; let m = node(E_MEMBER); m.a = e; m.s = eat_id(); e = m
       if e.a.kind == E_ID and e.a.s == "Regex" { g_uses_regex = true }   # splice the regex runtime on demand
+      if e.a.kind == E_ID and (e.a.s == "BigInt" or e.a.s == "Decimal") { g_uses_bignum = true }   # splice the bignum runtime on demand
       if e.a.kind == E_ID and e.a.s == "Query" { g_uses_query = true }   # splice the ECS spatial-query runtime on demand
       if e.a.kind == E_ID and e.a.s == "Reflect" { g_uses_reflect = true }   # force-emit the reflection ABI (Reflect.* reads the world schema)
       if e.a.kind == E_ID and e.a.s == "Light" { g_uses_light = true }   # splice the 2D light-accumulation pass on demand
@@ -421,6 +422,7 @@ function path_join(dir: pointer, rel: pointer) -> pointer {
 var loaded_paths: []pointer
 var cur_dir: pointer
 var g_uses_regex: bool = false    # a program mentioned Regex.* -> splice the regex runtime
+var g_uses_bignum: bool = false   # a program mentioned BigInt.*/Decimal.* -> splice the bignum runtime
 var g_uses_query: bool = false    # a program mentioned Query.* -> splice the query runtime + reflection ABI
 var g_uses_reflect: bool = false  # a program mentioned Reflect.* -> force-emit the reflection ABI
 var g_uses_light: bool = false    # a program mentioned Light.* -> splice the 2D light pass
@@ -599,6 +601,13 @@ function maybe_splice_runtime() -> void {
     do_import("runtime/native/regex_vm.ludic")
     cur_dir = saved
   }
+  # any program that uses BigInt.*/Decimal.* gets the bignum engine spliced in
+  # (self-contained — only compiler intrinsics — so a plain tool works too).
+  if g_uses_bignum {
+    cur_dir = ""
+    do_import("runtime/native/bignum.ludic")
+    cur_dir = saved
+  }
   # any program that uses Query.* gets the ECS spatial-query helpers spliced in;
   # they read entity state through the reflection ABI (emit_decl force-emits it
   # for a Query program even when it declares no events).
@@ -713,6 +722,7 @@ function parse_program() -> void {
   g_onlisten = new []Node
   g_toggled_layers = new []pointer
   g_uses_regex = false
+  g_uses_bignum = false
   g_uses_query = false
   g_uses_reflect = false
   g_uses_esys = false