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>

docs/language/ease/ease-elastic.md

@@ -0,0 +1,22 @@
+---
+id: ease-elastic
+name: Ease.elastic
+category: ease
+kind: namespace-method
+tokens: Ease.elastic
+sig: Ease.elastic(t) -> fixed
+tip: Ease out with a springy wobble.
+order: 5
+ns: Ease
+member: elastic
+---
+
+Returns an ease-out that shoots past <code>1.0</code> and wobbles around it a few times with shrinking amplitude before settling — a spring released under tension. The overshoot comes from a decaying <code>2<sup>-10t</sup></code> envelope on a sine, all in Q16.16, so it is deterministic and bit-identical on every platform. Great for UI that snaps in with personality: a panel that springs open, a picked-up item that jiggles, a selection that bounces to place.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let scale = Math.lerp(0.0, 1.0, Ease.elastic(progress))
+  }
+}
+```