Put the frame in a container with no decoder, and find the palette is not free

ROADMAP K2. DLXP1: a 49,664 B record that is 97 sectors exactly, no index and
no length word, because a packed record's length is geometry rather than
content. 582.0 KB/s, which is what FINDINGS 61.9 predicted to the tenth, and it
encodes in 3.3 s because there is no k-means in it.

px68k's own x68k/gvram.c renders the container's bytes index-exact with the
harness computing no interleave -- the only test that can catch an encoder whose
byte order is wrong, since a container round-trips against its own inverse
either way. Both negative controls fail as they must.

The picture is re-derived against this project's builder rather than PIL's
(34.05 dB against 61.9's 34.08) and the GGGGGRRRRRBBBBBI word is charged for the
first time in this tree: 0.53 dB, on every row, so it moves no comparison.

What the control found is the finding. A packed container on a SCENE palette
lands exactly on the codec's ceiling, so the whole +2.31 dB is the per-frame
palette and nothing else -- and 231 of 256 entries change every frame, which
makes a mismatched paint 12.8 dB worse than the correct pairing, on screen for
roughly half of every frame slot if buffer mode does not blank. So B2 now
decides which packed CONTAINER ships, not only which player. The fallback is
already a flag: --scene-palette --no-palette is 30.79 dB, zero churn, 576.0 KB/s
and still +2.07 dB on the shipping codec.

62.5 is priced and is a wash: palette first 20.32 dB, palette last 20.33.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6

tools/bench/gvpack/harness.c

@@ -97,6 +97,17 @@ int main(int argc, char **argv)
     int iw = (d[4] << 8) | d[5], ih = (d[6] << 8) | d[7];
     if (n < (size_t)(8 + 768 + iw * ih)) { fprintf(stderr, "short blob\n"); return 2; }
     const BYTE *pix = d + 8 + 768;
+    /* 'DLXQ' -- the blob is PRE-INTERLEAVED: `pix` is already the bytes a DLXP1
+     * record carries, in GVRAM order.  The ordinary 'DLXR' path computes the
+     * interleave here, which tests the LAYOUT; this path tests the CONTAINER,
+     * by writing its bytes verbatim and asking px68k's own gvram.c what they
+     * display as.  The two agreeing is the claim ROADMAP K2 has to make: the
+     * encoder's byte order is the one 47.2 verified as a picture. */
+    int prepacked = (d[3] == 'Q');
+    if (prepacked && !packed) {
+        fprintf(stderr, "a pre-interleaved blob has no unpacked form\n");
+        return 2;
+    }
     int yoff = (H - ih) / 2;
     const BYTE BLACK = 255;
 
@@ -121,8 +132,23 @@ int main(int argc, char **argv)
         for (int y = 0; y < H; y++) {
             DWORD base = 0xC00000 + y * 1024;
             for (int i = 128; i < 512; i++) wr16(base + i * 2, 0);
-            for (int i = 0; i < 128; i++)
-                wr16(base + i * 2, (WORD)((PIX(y, i + 128) << 8) | PIX(y, i)));
+            for (int i = 0; i < 128; i++) {
+                WORD w;
+                if (prepacked) {
+                    /* The letterbox rows are STATIC SETUP and are not in a
+                     * record (dlxp.py), so they are supplied here, the way a
+                     * player's scene setup supplies them: both halves BLACK. */
+                    if (y < yoff || y >= yoff + ih)
+                        w = (WORD)((BLACK << 8) | BLACK);
+                    else {
+                        const BYTE *row = pix + (y - yoff) * iw;
+                        w = (WORD)((row[i * 2] << 8) | row[i * 2 + 1]);
+                    }
+                } else {
+                    w = (WORD)((PIX(y, i + 128) << 8) | PIX(y, i));
+                }
+                wr16(base + i * 2, w);
+            }
         }
         if (!keepbuf) set_r20(R20_DISPLAY);       /* back to display */
     } else {