BLOCK_HEADER errors on 9 R2004 records of AutoCAD I.18 drawings: a 1,028-byte non-text description (§20.4.52) #386

Found by #384, which pins three new public sources for #375. Four drawings from Seeed-Studio/OSHW-Jetson-Series (Apache-2.0), saved by AutoCAD build I.18.0.0 as AC1018 (R2004), hold 9 BLOCK_HEADER (0x0031) records that error:

For these blocks the decoder reads a 1,028-byte non-text string as the block description. The R2004 tail of §20.4.52 (the inserted-entity count run, then the description, then the preview size) is then read from the wrong place, and the field list misses the record's boundary. The other blocks of the same drawings decode, and no other pinned drawing shows it.

#384's tools/corpus/baseline.tsv pins the 9 errors, so this is tracked rather than red. Whoever fixes src/tables/block_record.rs regenerates those rows. ARCHITECTURE.md section 7ar in #384 has the detail.

Done when

All 9 records close on their data-stream boundary, the fields that the drawings' DXF exports (if any) carry agree, and the corpus baseline shows 0 errored for these drawings. A finding about the field list (for example, a flag that gates the description, or a different count width) gets its own evidence table in ARCHITECTURE.md.