Adds SCN2681 devices at 0x0640000 (DUART1 = host/printer) and 0x0620000
(DUART2 = keyboard/mouse), gated on VT1200_DUART, with the remap
verified as duart_register = handler_off (duart_byte = off*2, value in
the low byte of the firmware's 16-bit move). Optional external TX->RX
loopback (VT1200_DUART_LB1/2) models the loopback connectors; the
monitor switch is driven on IP4-IP6. New VT1200_DUARTLOG gives a full
non-deduplicated DUART access log.
Result: the level-1 HOST PORT test is reached and executed against the
real DUART, but still reports 'LEVEL1/2 ERROR -> HOST PORT - 15'. The
test reads SRA/RHRA and requires a 5-byte external pattern
(0xAA,0xFF,0xAA,0x55) plus SRA overrun, yet the firmware never writes
the transmit register (0 writes to reg 3/B on either DUART in 1400
frames), so no internal or external loopback can supply it. See
harness/mame/DUART_REPORT.md.
Regression holds: no VT1200_* env -> PC=3FDCE400 samples=199.
The dispatch status is 0x100 (printed as '22' in the message). A driver-side
transparent low-RAM write log (VT1200_LOWLOG) shows the slot at bit 0x16F0 is
written by the store at 3FDF9090 from a13(0x10), which L3FA7C050 sets to the
literal 0x100 when the memory test L3F2018B0 returns non-zero. L3F2018B0 is the
level-1 video-memory test (base bit 0x04000000, count 0x20000). Its failing
checks: the 16-bit walking-bit read-back at 0x04000000 (baseline, unmapped) and,
with video RAM mapped, the REFCNT (0xC00001F0) 'must change' check.
Closed cross-check (single run): the catch-all and VT1200_CAL proxies are
directly comparable (both base 0) and resolve identically; last-wins holds
for same- and mismatched-width overlaps. The apparent conflict was a units
error: a .rw(FUNC(offs_t)) handler receives (bitaddr-base)>>4 (tms34010
Width=1, AddrShift=3), so the catch-all logs bitaddr>>4. Its 0x400000..0x40D7FF
is bit 0x04000000..0x040D7FF0 (bytes 0x800000..0x81AFFE), outside the declared
4 MiB RAM -- genuinely unmapped. The old 0x00400000 proxy was 16x too low.
Catch-all reproduces the committed log byte-for-byte and does not perturb
coverage (56 / PC=3F208950 with and without). Regression holds: PC=3FDCE400
samples=199. Report: harness/mame/XCROSS_REPORT.md
SDD