feedback7 R22-RENDER: a pushed / pulled enemy slides to where the sim put it, a dying flyer keeps its lift through the Die clip, a long Die clip plays whole (PR #380 by @xcdoge); golden: unchanged

The sim displaces at once (battle/displacement.js), so the snapshots only show the destination: a ≤ 2.5-tile push was
lerped over one snapshot interval (3 frames, 0.58 tiles in one frame — frame-loop repro over a real battle), a longer
pull snapped. render/app.js now starts UnitView.slideTo as soon as the interval it samples ends on the snapshot that
carries the destination (a `displace` fx still queued up to SnapshotBuffer.nextSnapT); the view holds where it stood
until that snapshot is shown, then eases into the sampled position under a constant deceleration (v0 = 2D/T, a = v0/T,
T = 0.14·√D in 0.12–0.45 s — ours, [ASSUMED]: no source gives the official timing; docs/research/13 has the official
impulse-and-friction classes). A unit that dies on the way finishes the slide and lies where the push put it.
Death: DIE_CLIP_MAX 1.6 → 8 s (data/assets.json: 234 enemy Die clips, the longest 7.97 s, 25 over 1.6 s), and a
flyer holds FLY_HOVER until the fade tail (DIE_FADE_TAIL 0.55 s).

Substantially @xcdoge's code (PR #380: the slide, the death timing, research 13 / 14, most tests), authored as theirs.
Changed on review: the corpse no longer goes back to where it stood (the PR's test asserted that; the sim's body is at
the destination); the PR's pre-pass scanned (renderT − 0.5, renderT] after processEvents had taken those events, so
it never fired and the slide started a frame before landing — it now looks ahead to the next snapshot and holds the
view until then; the slide eases into the live sample instead of the fixed destination (no jump back onto an enemy
that walked on); each step uses its mean velocity (the end-of-step speed ran 9 % short, then jumped); the constants'
comments say the timing is ours; research 14 no longer cites a sweep script and a data guard that were not in the PR.

Golden: unchanged (client only).

Tests: test/render/unitview.test.js 35/35 (slide: deceleration, idempotence, facing, snapshots in flight, friction,
√D, corpse lies where pushed, hold-then-ease without a landing jump, a never-shown slide handed back; the app.js frame
order over a real battle: a 1.7-tile push over ≥ 8 frames with every step < 0.35 tiles, a push-kill's body staying at
the destination; death: a dying flyer's lift, every Die clip within DIE_CLIP_MAX and a 7.97 s clip scheduled whole);
test/sim/displace-fx.test.js 1/1; render/interp 16/16; docs tests pass.

public/js/render/interp.js

@@ -283,6 +283,16 @@ export class SnapshotBuffer {
     return ans;
   }
 
+  /**
+   * Game time of the first snapshot after `time` (default renderT) — the newer end of the interval sample() interpolates
+   * — or NaN when there is none (extrapolating). render/app.js starts a push's slide while that interval is shown.
+   */
+  nextSnapT(time = this.renderT) {
+    const s = this.snaps;
+    const i = this._indexAt(time) + 1;
+    return Number.isFinite(time) && i < s.length ? s[i].t : NaN;
+  }
+
   /**
    * The `down` list (knocked-out operators waiting to redeploy, see header) of the snapshot shown at `time` (default
    * renderT) — the same snapshot sample() reads flags from — or null.