body: address CodeRabbit review — heart-as-solid, gzip sniff, LOD retry, a11y

CodeRabbit findings on #64:
- Heart no longer classified as vessel material 0: it's a chambered organ, so the
  slicer-fill was sweeping a PCA-centerline lumen rod through it. Now falls through
  to solid_tissue (22 concepts artery→solid; fill 715→693 components). Wire + LOD
  blocks rebaked + reuploaded.
- Decompress by the payload, not DecompressionStream support: sniff the gzip magic
  (1f 8b) so a browser/CDN that already decoded Content-Encoding doesn't get
  double-inflated, and a browser without DecompressionStream errors clearly instead
  of feeding gzip bytes to the decoder.
- Server-LOD no longer permanently disabled after one failed poll: toggling LOD off
  clears the failure flag so re-enabling retries.
- Search-result rows are now keyboard-focusable <button type="button"> elements
  instead of mouse-only <div>s.
- Plan doc: marked the BF16 (ver 4) section SUPERSEDED by F16 (ver 5) to match the
  shipped wire.

Author: claude

claude-notes/plans/2026-06-27-body-v3-server-lod-fill.md

@@ -106,15 +106,22 @@ centroid (robust to outliers), clamped to an **absolute** `[RMIN, RMAX]`
 diameter boundary (`RMAX=0.020` ≈ 34 mm dia, covers the aorta, kills balloons).
 662 vessels → +71,872 core verts / +133,152 tris.
 
-### BF16 positions — BSO2 **ver 4** (the "A" brick)
-Per-vertex `pos` column is now **BF16** (3× u16 LE = 6 B/vertex), half of ver 3's
+### Half-precision positions — the "A" brick
+> **SUPERSEDED to F16 (BSO2 ver 5).** BF16 (ver 4) was tried first and **rejected**:
+> its 7-bit mantissa gave a ~3 mm step near the head (y≈0.85) → a visible staircase
+> (Treppeneffekt) on the eye/brain. Shipped format is **F16 / IEEE half (ver 5)** —
+> same 6 B/vertex, 10-bit mantissa, ~0.2 mm (measured 0.21 mm max over the wire), no
+> staircase. Bake uses ndarray's `F16::from_f32`; renderer widens via a 64K half→f32
+> LUT. `BodyV3.tsx` reads ver 3 (f32) / 4 (BF16) / 5 (F16). gz ≈ 63 MB (vs f32's 80).
+> The BF16 description below is retained for history.
+
+Per-vertex `pos` column was **BF16** (3× u16 LE = 6 B/vertex), half of ver 3's
 12 B f32. Conversion via ndarray's sanctioned RNE batch path
 (`f32_to_bf16_batch_rne`) on the native bake host (AVX-512/AMX). The renderer
 widens back to f32 client-side (`bits << 16` — BF16 is the top 16 bits of f32, so
-the widening is exact). Verified round-trip ≈ 1.4 mm error on a 1.7 m body — below
-the visual + cascade (screen-space-error) floor. Wire 176 MB → 150 MB; gz 80 MB →
-57.5 MB. `BodyV3.tsx` reads both ver 3 and ver 4. Asset replaced in release
-`fma-body-soa-v3-v1` (Dockerfile pulls it same-origin, unchanged).
+the widening is exact). Round-trip ≈ 1.4 mm — which turned out to be visible on
+small smooth structures, hence the F16 upgrade above. Asset in release
+`fma-body-soa-v3-v1` (Dockerfile pulls it same-origin).
 
 ### "B": server-side HHTL-O(1) LOD endpoint — WIRED (de-risked)
 cockpit-server can't build in-sandbox (quarto-core→runtimelib is a proxy-blocked

cockpit/src/BodyV3.tsx

@@ -301,6 +301,7 @@ function mount(container: HTMLDivElement, d: Decoded, st: RenderState, onStats:
     if (renderer.getPixelRatio() !== pr) renderer.setPixelRatio(pr);
     uniforms.uEnabled.value = st.enabled;          // shared by both materials
     uniforms.uGlobalAlpha.value = st.alpha;
+    if (!st.lodOn) lodFail = false;   // toggling LOD off clears a transient failure → re-enabling retries
     uniforms.uLodOn.value = st.lodOn && !lodFail && lodReady ? 1 : 0;   // only cull after a real response
     if (st.focus) {                                 // search-pick zoom: glide to the organ
       tmp.set(st.focus.t[0], st.focus.t[1], st.focus.t[2]);
@@ -360,10 +361,18 @@ export function BodyV3() {
 
   useEffect(() => {
     let cancelled = false;
-    const inflate = async (r: Response) =>
-      r.body && typeof DecompressionStream !== 'undefined'
-        ? new Response(r.body.pipeThrough(new DecompressionStream('gzip'))).arrayBuffer()
-        : r.arrayBuffer();
+    // Decide by the *payload*, not by DecompressionStream support: if the browser (or a
+    // CDN) already decoded Content-Encoding: gzip, the bytes are plain; if served raw,
+    // they start with the gzip magic 1f 8b. Only inflate when actually gzipped.
+    const inflate = async (r: Response): Promise<ArrayBuffer> => {
+      const buf = await r.arrayBuffer();
+      const u8 = new Uint8Array(buf);
+      const gz = u8.length > 1 && u8[0] === 0x1f && u8[1] === 0x8b;
+      if (!gz) return buf;
+      if (typeof DecompressionStream === 'undefined')
+        throw new Error('body.soa.gz is gzip but this browser lacks DecompressionStream');
+      return new Response(new Blob([buf]).stream().pipeThrough(new DecompressionStream('gzip'))).arrayBuffer();
+    };
     (async () => {
       const local = await fetch('/body.soa.gz').catch(() => null);
       if (local && local.ok) return decodeBso2(await inflate(local));
@@ -428,10 +437,10 @@ export function BodyV3() {
           {matches.length > 0 && (
             <div style={{ marginTop: 4, background: '#0e1219', border: '1px solid #1c2530', borderRadius: 6, overflow: 'hidden', maxHeight: 320, overflowY: 'auto' }}>
               {matches.map((c) => (
-                <div key={c.row} onClick={() => { pick(c); setQuery(''); }} style={{ padding: '6px 9px', cursor: 'pointer', display: 'flex', justifyContent: 'space-between', gap: 8, borderBottom: '1px solid #141b24', color: '#cdd9e5' }}>
+                <button type="button" key={c.row} onClick={() => { pick(c); setQuery(''); }} style={{ width: '100%', textAlign: 'left', padding: '6px 9px', cursor: 'pointer', display: 'flex', justifyContent: 'space-between', gap: 8, border: 'none', borderBottom: '1px solid #141b24', background: 'transparent', color: '#cdd9e5', font: '13px ui-monospace, monospace' }}>
                   <span style={{ overflow: 'hidden', textOverflow: 'ellipsis', whiteSpace: 'nowrap' }}>{c.name}</span>
                   <span style={{ opacity: 0.5, flexShrink: 0 }}>{LAYERS[(c.layer - 1) % 8]?.name}</span>
-                </div>
+                </button>
               ))}
             </div>
           )}

crates/cockpit-server/assets/body.blocks

@@ -48,8 +48,10 @@
     {"id": 5, "name": "neural", "doppler": "none", "rgb": [226, 205, 88]},
 ]
 TISSUE_MATERIAL = {
-    "artery": 0, "heart": 0, "vein": 3, "vessel": 3, "nerve": 5,
-    # everything solid (bone/cartilage/muscle/organs/skin/flesh) → solid_tissue
+    "artery": 0, "vein": 3, "vessel": 3, "nerve": 5,
+    # heart is a chambered ORGAN, not a tube — mapping it to artery(0) made the slicer-
+    # fill sweep a PCA-centerline lumen rod through it. Fall through to solid_tissue
+    # (like every other solid: bone/cartilage/muscle/organs/skin/flesh).
 }