t8ad: a surface is a list of sub-rectangles, not a 256 grid -- disc decode 96% -> 100%

The "~15% deferred variants" were not variants. Auditing every T8aD on the disc
gave 19216 surfaces, 18442 decoding (96.0%) and 774 failing in two clusters:
GP_DIALOG strips declaring 524x63 with a "tile count" of 1 or 2 instead of 3, and
small textures in the six *2D language paks whose pixels ran past the end of the
file.

Both fall out of the per-tile header, which is not opaque flags: it is four BE
u32 -- dst X, dst Y, width, height. A 15x18 icon stores a 13x18 rectangle at
(1,0); pdmes010 stores (59,6,256,54) and (315,6,149,54), the second beginning
exactly 16 + 256*54*4 bytes after the first. So 0x1c is a RECTANGLE COUNT and the
256-grid reading was an accident of most surfaces being stored as full-width
bands.

Parser rewritten to that model, still refusing to guess: a rectangle must fit the
declared surface and its pixels must fit the file, else None. Disc decode is now
19216/19216 = 100.00%.

Two test fixtures were built to the old model and are corrected rather than
worked around. lsta's t8ad_frame wrote NO offset-table entry, so the decoder read
"pixels" from inside the header -- the test passed only because it checked
dimensions alone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/sylpheed-formats/src/lsta.rs

@@ -48,12 +48,21 @@ pub fn parse(bytes: &[u8]) -> Option<Vec<T8adImage>> {
 mod tests {
     use super::*;
 
+    /// A faithful one-rectangle T8aD frame: base header, a 1-entry offset table,
+    /// then the rectangle header (dst 0,0, size w×h) and its pixels. (Before
+    /// 2026-08-11 this fixture wrote no offset-table entry at all and the decoder
+    /// read "pixels" from inside the header — the test only ever checked the
+    /// dimensions, so it passed anyway.)
     fn t8ad_frame(w: u32, h: u32) -> Vec<u8> {
-        let mut b = vec![0u8; 64];
+        let mut b = vec![0u8; 0x2c];
         b[0..4].copy_from_slice(&T8AD_MAGIC);
         b[0x14..0x18].copy_from_slice(&w.to_be_bytes());
         b[0x18..0x1c].copy_from_slice(&h.to_be_bytes());
         b[0x1c..0x20].copy_from_slice(&1u32.to_be_bytes());
+        b.extend_from_slice(&0x30u32.to_be_bytes()); // offset table → rect at 0x30
+        for v in [0u32, 0, w, h] {
+            b.extend_from_slice(&v.to_be_bytes()); // dst X, dst Y, width, height
+        }
         b.extend_from_slice(&vec![0x80u8; (w * h * 4) as usize]);
         b
     }