build-lance.sh: fail the build if .text overlaps the marker block

The text panel grew .text past MARK and the payload overwrote its own code
with cell() writes (illegal opcode on the unit). The build now parses
build/lance.map and aborts if the end of .text reaches MARK, printing the
remaining gap.

harness/udpstream/build-lance.sh

@@ -87,6 +87,24 @@ done
 OUTNAME=${LANCE_OUT:-lance.coff}
 cp "$WORK/LANCE.OUT" "$OUT/$OUTNAME"
 sed 's/\r$//' "$WORK/LANCE.MAP" > "$OUT/lance.map"
+# Guard: the marker block (MARK in the source) must sit clear of .text, or the
+# payload overwrites its own code with cell() writes (an illegal-opcode crash).
+python3 - "$SRC" "$OUT/lance.map" <<'PYCHK'
+import re, sys
+src, mp = sys.argv[1], sys.argv[2]
+m = re.search(r'#define\s+MARK\s+(0x[0-9A-Fa-f]+)', open(src).read())
+if m:
+    mark = int(m.group(1), 16)
+    for ln in open(mp):
+        f = ln.split()
+        if len(f) >= 4 and f[0] == '.text':
+            start, size = int(f[2], 16), int(f[3], 16)
+            end = start + size
+            if end > mark:
+                sys.stderr.write("build: ERROR .text ends at 0x%X, past MARK 0x%X\n" % (end, mark))
+                sys.exit(1)
+            print("build: .text ends 0x%X, MARK 0x%X (%d bytes clear)" % (end, mark, (mark - end) // 8))
+PYCHK
 sed 's/\r$//' "$WORK/LANCE.LST" > "$OUT/lance.lst"
 sed 's/\r$//' "$WORK/ENTRY.LST" > "$OUT/entry.lst"
 echo "build.sh: $OUT/$OUTNAME ($(wc -c < "$OUT/lance.coff") bytes)"