Three w6a defects, one component. MOVB with an SP/BP/SI/DI register
operand encoded AH/CH/DH/BH -- emit_rex suppressed the bare 0x40
that selects SPL/BPL/SIL/DIL (the comment claimed "we always emit
REX"; encode probe: `MOVB SI,(AX)` -> `88 30` = %dh). emit_rex8
forces the byte for low-byte codes 4-7; no current w6c output emits
those shapes, so all existing objects are unchanged. LEAQ and CALL
unsupported operand shapes fell through with zero bytes and no
errs++ (every MOV arm reports); both now err. Every hardcoded
os.write diagnostic length in the wwstage was one byte short
(truncating the newline/quote); all converted to the .len idiom so
the length cannot drift again.
Hojun-Cho
06c40a8bef