docs: next level-1 gate is HOST PORT (window 0x6400000); evidence and approach recorded

harness/mame/BRINGUP_LOG.md

@@ -987,3 +987,37 @@ condition it fails on, model it, re-run the console capture.
 1. Model the host DUART (DUART1 channel A) enough for the level-1 HOST PORT test
    (bit 0x3F...); the DUARTs are unmodelled and are the current gate.
 2. Keep using cap.lua to read which test fails next -- the loop is now repeatable.
+
+## Next gate: level-1 HOST PORT (DUART1 channel A) — evidence so far
+
+With video memory mapped and REFCNT advancing, the level-1 sequence advanced to:
+
+```
+'LEVEL1/2 ERROR -> HOST PORT - 15 '      (error-string file 0x17938D, handler 0x07FBD3C2)
+```
+
+This is DUART1 channel A per the service guide's level-1 list — a local-loopback test on the
+host serial port. The DUARTs are **not modelled**, so it fails.
+
+**Evidence** (`VT1200_BOARD_LOG`, 700 frames, with the video-memory fixes in place) — the
+most-touched unmodelled register during that window:
+
+```
+31 W  06400002        <- by far the hottest; a DUART register being written repeatedly
+13 W  0620000A    13 W  06200002
+12 W  0640000A     6 R  0640000D     6 R  06200001
+```
+
+So window **`0x6400000`** (the driver's address units; offset `0x02`) is where the host-port
+test lives. A loopback test writes the transmit register and reads the receive register, so
+the next step is to find which offsets it reads back and give the window DUART-like
+behaviour.
+
+**Preferred approach:** wire MAME's `SCN2681`/`MC68681` device at that window (it is the
+right part, and MAME has it), rather than hand-rolling a loopback. Fallback: a stateful
+transparent register file in the driver with write-to-TX appearing at RX.
+
+**Context for scheduling:** level 1 is 15 tests. One now passes (video memory). The method is
+settled and repeatable — identify the test from the console, find the condition, model it,
+re-run — but each remaining gate (DUARTs, keyboard, mouse, NVRAM, LANCE, video RAM) needs a
+modelled device. Level 0 already passes.