feat(packages): ludic.anim - glTF animation clips read out of the parsed document, sampled by halving, cross-faded two at a time and folded into a render3d Skin's pose; clips per rig by name, translations off unless asked, a late first key reported

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

packages/ludic.anim/mix.ludic

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+# mix.ludic - sample one clip at a time into per-node buffers, mixed in at a weight. `rot` is 4 per
+# node, `pos` 3 per node, and `hit` says which nodes have received a value (bit 0 a rotation, bit 1
+# a translation), so a node the clips never mention stays at rest
+var q_a: floats = null
+var q_b: floats = null
+var q_cur: floats = null
+
+export function anim_mix(c: Clip, sk: Skin, t: float, w: float, rot: floats, pos: floats, hit: words) -> void {
+  if c == null or sk == null { return }
+  if q_a == null {
+    q_a = floats(4)
+    q_b = floats(4)
+    q_cur = floats(4)
+  }
+  var tt = t
+  if c.dur > 0.0 { tt = t % c.dur }
+  for ch in 0 .. c.n {
+    let node = c.cnode[ch]
+    if node < 0 or node >= sk.n_nodes { continue }
+    let n = c.ckeys[ch]
+    let o = c.coff[ch]
+    let i0 = anim_key_at(c, ch, tt)
+    var i1 = i0 + 1
+    if i1 > n - 1 { i1 = n - 1 }
+    var f = 0.0
+    if i1 > i0 {
+      let span = c.times[o + i1] - c.times[o + i0]
+      if span > 0.000001 { f = Math.clamp((tt - c.times[o + i0]) / span, 0.0, 1.0) }
+    }
+    if c.cpath[ch] == AC_ROT { mix_rot(c, ch, node, i0, i1, f, w, rot, hit) } else { mix_pos(c, ch, node, i0, i1, f, w, pos, hit) }
+  }
+}
+
+# Across clips the first contributor lands and the rest are nlerped onto it by their share, which
+# is what makes a cross-fade rather than a jump. Test the ROTATION bit, not the whole word: glTF
+# writes a node's translation before its rotation, and `hit == 0` then skipped every rotation.
+function mix_rot(c: Clip, ch: int, node: int, i0: int, i1: int, f: float, w: float, rot: floats, hit: words) -> void {
+  let vo = c.cvoff[ch]
+  for e in 0 .. 4 {
+    q_a[e] = c.vals[vo + i0 * 4 + e]
+    q_b[e] = c.vals[vo + i1 * 4 + e]
+  }
+  q_nlerp(q_a, q_a, q_b, f)
+  if (hit[node] & 1) == 0 {
+    q_store(rot, node, q_a)
+    hit[node] = hit[node] | 1
+  } else {
+    q_load(q_cur, rot, node)
+    q_nlerp(q_cur, q_cur, q_a, w)
+    q_store(rot, node, q_cur)
+  }
+}
+
+function mix_pos(c: Clip, ch: int, node: int, i0: int, i1: int, f: float, w: float, pos: floats, hit: words) -> void {
+  let vo = c.cvoff[ch]
+  let ax = Math.lerp(c.vals[vo + i0 * 3], c.vals[vo + i1 * 3], f)
+  let ay = Math.lerp(c.vals[vo + i0 * 3 + 1], c.vals[vo + i1 * 3 + 1], f)
+  let az = Math.lerp(c.vals[vo + i0 * 3 + 2], c.vals[vo + i1 * 3 + 2], f)
+  if (hit[node] & 2) == 0 {
+    pos[node * 3] = ax
+    pos[node * 3 + 1] = ay
+    pos[node * 3 + 2] = az
+    hit[node] = hit[node] | 2
+  } else {
+    pos[node * 3] = Math.lerp(pos[node * 3], ax, w)
+    pos[node * 3 + 1] = Math.lerp(pos[node * 3 + 1], ay, w)
+    pos[node * 3 + 2] = Math.lerp(pos[node * 3 + 2], az, w)
+  }
+}