0.2.6-dev: merge claude/r37-impostor — GitHub #542: impostor boxes from each skeleton's measured clip bounds (idle core, played clips within limits, the rest drawn directly while they play), every slot drawn inside its own scissor, frozen models posed at their clip's first frame, PIXI's pooled batch buffers (no Apple GPU stall from the per-slot flushes); golden: unchanged

# Conflicts:
#	CHANGELOG.md
#	docs/design/client.md
#	docs/history/0.2.6.md

docs/history/0.2.6.md

@@ -105,6 +105,79 @@ counts order entries absent from `draft.picks`, alongside the existing turn and
 `skipBand` already rejects the final player with `BAD_TARGET / nobody to pass to`; its rule remains unchanged.
 Regression covers normal two-, three- and four-human browser flows, the final turn, and exhausted quota.
 
+### 31.6 Impostor bounds and slot clearing (#542)
+
+`render/spine.js clipBounds / stepClipBounds / SpineActor.update`, `render/units.js impostorBox / _impBox / _impBypass`,
+`render/impostor.js`, `render/app.js`.
+
+- **Seen** (GitHub #542 by @ImPQlc; quality 高, usually from round 7 on): 野鬃's flag cut, offset and leaving stray pixels
+  while she idles; 浊心斯卡蒂's head cut in her S3 flight (Skill_3_Begin); 隐现's pistol muzzle cut, offset and leaving stray
+  pixels during her skill.
+- **Cause** (reproduced in headless Chrome: the dev demo's stress crowd, 120 units at impostor interval 4, the three
+  held in their widest poses): a unit's impostor slot was sized once from the pose it showed when it first became an
+  impostor, plus a margin (`_impBox`: the pose's bounds + 30 % / 22 %). A later pose past that box was drawn past the
+  slot's edge: cut in the unit's own sprite and painted into whatever lay beside the slot on the atlas page. A page is
+  cleared on its first flush only and each eraser covers its own slot, so a spill into free space stayed, and a spill
+  into another unit's slot showed on that unit until it was redrawn in a frame without the spill. 浊心斯卡蒂's flight reaches
+  y −704 (skeleton units) against a box top near −516 (54–57 px past her slot in 5 of 5 runs); 野鬃's idle flag reaches x
+  −434, 103 units past a box sized from her setup pose (0–47 px past her slot over 5 runs: it depended on that first
+  pose); after one clean redraw of every impostor the atlas pages held 0–1,405 pixels outside every live slot. 隐现's own
+  poses fit her box (the silencer reaches x 197, her box 225): her muzzle sits at her slot's edge, where a neighbour's
+  spill lands. Of the roster's 681 skeletons, 598 play a clip that leaves a box sized that way from their idle's first
+  frame. No clipping attachment is involved (`spine.js _eyeMaskFallback`, #177, unchanged).
+- **Now**: (1) every clip a unit can play (its manifest roles with every skill's set and the Run cycle, its `FORMS` clip
+  sets and change clips) is measured once per skeleton data on a scratch skeleton: 60 poses a second, at most 49 per
+  clip; a region by its attachment rectangle, a mesh by its vertices; slots drawn at alpha 0 skipped. It is queued when
+  the model loads and sampled within 2.5 ms per frame (`stepClipBounds`, called by `render/app.js`), ≈ 9 ms per skeleton
+  on the roster (53 ms at most), shared by every unit of that skeleton; a unit is drawn directly until its box is ready.
+  A skeleton stays queued only while an actor uses it: when the last one is destroyed (a model swap, a battlefield torn
+  down) its pending clips and its scratch skeleton are dropped, so nothing holds a skeleton the Spine store let go or is
+  measured ahead of the next scene's models, and a later actor queues again what was never measured. (2) The box is the
+  idle clip, then each other clip in the order of the area it adds while the box stays within 5 × the idle clip's area
+  (a looping or held clip) or 2 × (a one-off clip: the deploy clip, a skill's, stun's or move's begin and end, a form's
+  change), padded by 3 % + 8 units. A clip that does not fit (`over`: 355 of the roster's 681 deploy clips — the drop
+  from above, up to 1,700 units tall — and the looping or held clips of 13 skeletons with screen-wide effects, among
+  them 帕拉斯's skills and 诗怀雅's attack) is drawn directly while it plays; the unit keeps its slot (`_impBypass`). So is a
+  clip whose sampling threw (recorded as not measurable, never as drawing nothing — the scissor would cut it). The box
+  is computed once per unit: no bounds work per frame. The setup pose needs no room: a model frozen before its first
+  update — built while its unit is stunned and it has no stun clip (202 of the 210 operators have none), or dead without
+  a death or idle clip — showed its setup pose, which no clip shows (隐现's Back model stands 900 units right of its feet
+  in it); `SpineActor.update` now poses it once at its clip's first frame (again on the next frame if that update
+  threw). (3) The atlas draws each slot's skeleton in a container masked by the slot rectangle, which PIXI draws as a
+  scissor rect (no stencil, no render-pass break): nothing paints outside a slot, and the slot's eraser clears all that
+  an earlier pose can have left in it. Each redrawn slot is a batch flush of its own, and PIXI uploaded its one batch
+  vertex buffer again for every flush, which stalls Apple GPUs (Chrome, ANGLE Metal, Apple M4: the 120-unit crowd at
+  1920 × 1080 ran at 42–58 fps instead of 60, by the machine's load), so `render/app.js` turns PIXI's
+  `BatchRenderer.canUploadSameBuffer` off: a buffer per flush, PIXI's own setting on iOS. Switched back and forth within
+  one page it gave 60 fps against 42 (DPR 2) and 50 (DPR 1) with the shared buffer — turning it off only around the
+  atlas pass gave 55 — for ≈ 0.3 ms more render CPU per frame; under load it also steadied the 0.2.5 renderer (51 → 59
+  fps at DPR 2).
+- **Cost**: slot sizes grow with the boxes. On the roster they add up to 1.26 × the old area (the median skeleton 1.13
+  ×). The 120-unit stress crowd at 1600 × 900 (DPR 1) uses 5.5 M px of slots instead of 4.4 M (two of the three atlas
+  pages, no private render target). At DPR 2 a page holds a quarter of the CSS pixels: the crowd at 1280 × 720 keeps
+  98–108 of its units in the atlas instead of 118–120, at 1920 × 1080 59–61 instead of 58–59 (its private render targets
+  4.4 M px instead of 2.8 M); at 844 × 390, DPR 3, all 120 stay in two pages. 60 fps in every case, before and after
+  (runs on a quiet machine). A unit that finds no slot keeps a private render target, as before; the atlas keeps its ≤ 3
+  + 2 pages. CPU: `tools/perfbench.mjs` (844 × 390, DPR 2, quality medium / low, where those battles draw through the
+  atlas; four battles at a 4 × CPU throttle, two interleaved runs before and after): the view's JS 4.69 → 5.04 ms per
+  frame, the impostor-redraw stage +21 % (+0.12 ms per frame), all busy JS +0.4 %, 60 fps in every run.
+- [ASSUMED]: the growth limits (5 × / 2 ×), the 3 % + 8 padding, the sampling rate and the 2.5 ms budget. A pose between
+  two samples can still pass the padded box for a frame (42 of the 681 skeletons by a denser 60 Hz check, all in long
+  clips sampled sparser than that); the scissor cuts it and leaves nothing behind.
+- Tests: `test/render/impostor-bounds.test.js` (synthetic Spine 3.8 skeletons: clip bounds, an attachment shown for 0.05
+  s, an alpha-0 slot, the budgeted queue, a skeleton leaving the queue with its last actor, a clip whose sampling
+  throws; a model frozen before its first update — stunned without a stun clip, dead without a death or idle clip —
+  posed once at its clip's first frame, again after an update that threw; `impostorBox` / `playedClips`; every atlas
+  slot's clip box and eraser; UnitView: drawn directly until measured, built while stunned, a skeleton without an Idle
+  clip, the over-clip bypass keeping its slot, an unmeasurable attack or idle clip drawn directly, the legacy fallback,
+  a pen figure), `test/render/impostor-bounds-models.test.js` (the three operators through the real runtime: every pose
+  of every clip they play, at 30 Hz, inside their box; the old box cut 野鬃 by 103 units and 浊心斯卡蒂 by 188; 隐现's Back model
+  built while stunned shows its Idle's first frame inside the box, its setup pose lies outside it; skips without the
+  assets), `test/render/impostor-bounds.browser.test.js` (`RENDER_E2E=1`: the held stress crowd — inside their slots, no
+  atlas pixel outside a live slot; the scissor on a bare page; 野鬃's deploy drawn directly),
+  `test/render/boardart.test.js` (the allocator's test doubles). Dev: `render-demo.html?scene=stress&ops=<chessId>,…`.
+  Golden: unchanged.
+
 ### 31.7 Official scene light gains (#549)
 
 GitHub #549 (@xcdoge, confirmed by other players): the official battlefield scenes of 0.2.5 read much darker than the