feat(stdlib): Math.exp/log/pow + Ease.elastic transcendentals (issue #25)

Finish the unblocked "Math / Ease" half of #25: the fixed-point
transcendentals deferred from #2. Vec.* stays blocked on the vec2 type
in #1.

- Math.exp, Math.log (natural), Math.pow — deterministic Q16.16 via two
  new prelude fns in emit_math_prelude: @fn_fx_exp2 (range-reduced 5th-order
  Taylor 2^f, then a clamped shift by the integer part) and @fn_fx_log2
  (llvm.ctlz for the exponent + an atanh series on (m-1)/(m+1) for the
  mantissa). exp=2^(x·log2 e), log=log2(x)·ln2, pow=2^(b·log2 a).
- Ease.elastic — ease-out elastic 2^(-10t)·sin((10t-0.75)·2pi/3)+1.
- Pure integer IR, so bit-identical on every platform. Results must fit the
  Q16.16 range (|x| < 32768); larger magnitudes saturate (documented).

Test selfhost/tests/transcend.ludic (registered in the self-host suite) +
docs for all four. Reseeded; the C-free bootstrap fixpoint holds. All suites
green (24 self-host, 45 regression, 29 tool).

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

tools/x/selfhost_test.ludic

@@ -45,6 +45,7 @@ fn cmd_selfhost_test() -> int {
   sh_case("easecollide", "250 750 500 1000 1000 999 765 -88 1000 1 0 1 0 0 1 0 1 0")
   sh_case("memsys", "65 66 0 99 12345 1")
   sh_case("math3", "45 90 180 -135 30 60 90")
+  sh_case("transcend", "2718 1 148 1000 0 2303 1024 1414 3 250 0 1000 1016")
   sh_case("textsplit", "1 1 1 3 1 1 1 1 1 1 1")
   sh_case("hash", "-2128831035 -468965076 114400290 114400290 0 -873187034 1095738169 0 1364076727 -2114883783 -845898438 -845898438 -78065325 -78057399 -3750763034362895579 -5808556873153909620 -4100651535478758590 -4100651535478758590 0 -5451962507482445012 7256831767414464289")
   sh_case("long", "1000000000000 1000000000001 1000000000005 3000000000000 1 1 1 -1000000000000 1000000 13")