Add a Lerp node that blends from a to b with an ease (#246)

Every animation in a behavior had to build its interpolation from
Subtract, Multiply and Add, once per axis. The gold pickup in
"Garden of Sacrifices" needed about 30 nodes for that alone. A single
Lerp node with an ease makes those chains one node each.

- Lerp has the inputs a (default 0), b (1) and t (0) and the output
  result. It sits in the data category next to Clamp.
- The Ease setting offers Linear, In, Out and In-out, all quadratic.
  The node row shows it as "Ease: Out".
- t is held in 0..1. A timer that overshoots in the last frame still
  lands exactly on b.

Tests:
- ./check.sh: 178 files, 2583 tests, CHECK EXIT 0
- 7 new cases in the node catalog test. Without the clamp on t,
  2 of them fail.
- e2e 6z-iii (2 checks): typing "lerp" in the palette adds a linear
  Lerp, and picking "Out" in the inspector reaches the node and its
  row. GM_STEPS=1-6: ALL PASSED, 267 checks.
- README check sum is 1459, computed, not counted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/node-catalog.test.ts

@@ -556,6 +556,30 @@ const CASES: Record<string, Case[]> = {
     { name: 'above the range', inputs: { value: 5, min: 0, max: 1 }, outputs: { result: 1 } },
     { name: 'below the range', inputs: { value: -5, min: 0, max: 1 }, outputs: { result: 0 } },
   ],
+  Lerp: [
+    { name: 'linear by default', inputs: { a: 10, b: 20, t: 0.25 }, outputs: { result: 12.5 } },
+    { name: 'ease in', inputs: { a: 0, b: 8, t: 0.5 }, params: { ease: 'in' }, outputs: { result: 2 } },
+    { name: 'ease out', inputs: { a: 0, b: 8, t: 0.5 }, params: { ease: 'out' }, outputs: { result: 6 } },
+    {
+      name: 'ease in-out, slow at both ends',
+      inputs: { a: 0, b: 8, t: 0.25 },
+      params: { ease: 'inOut' },
+      outputs: { result: 1 },
+    },
+    {
+      name: 'ease in-out, half way at the middle',
+      inputs: { a: 0, b: 8, t: 0.5 },
+      params: { ease: 'inOut' },
+      outputs: { result: 4 },
+    },
+    { name: 't past 1 stops at b', inputs: { a: 3, b: -5, t: 1.2 }, outputs: { result: -5 } },
+    {
+      name: 't below 0 stops at a',
+      inputs: { a: 3, b: -5, t: -0.5 },
+      params: { ease: 'out' },
+      outputs: { result: 3 },
+    },
+  ],
   Floor: [
     { name: 'rounds down', inputs: { value: 2.9 }, outputs: { result: 2 } },
     { name: 'rounds down for negatives too', inputs: { value: -2.1 }, outputs: { result: -3 } },