harness: explicit monochrome palette (pen 0 black, pen 1 white)

vt1200.cpp declared PALETTE(config, m_palette).set_entries(256) with no init
delegate, so render_active_line()'s pens 0/1 rendered in whatever MAME's
default palette held -- reported as red on screen.  Use
palette_device::MONOCHROME (emupal.cpp: palette_init_monochrome) with two
entries, so the colour is ours and deterministic.

Verified via a real MAME screenshot (screen:snapshot(), which renders through
the palette -- dump_frame()'s PPM is straight from bitmap pens and cannot show
colour): pal_mono_frame600.png is exactly black (0,0,0) + white (255,255,255).
palverify.lua takes the screenshot; the frame is the level-1 error screen.

harness/mame/palverify.lua

@@ -0,0 +1,30 @@
+-- palverify.lua -- take a real MAME screenshot (video manager -> palette applied)
+-- at a chosen frame.  Unlike dump_frame()'s PPM (straight from bitmap pens), a
+-- screen:snapshot() renders through the palette, so it is the correct instrument
+-- for checking the mono colour ramp.
+--
+--   PAL_FRAME=<n>  PAL_OUT=<abs path .png>  <mame> ... -autoboot_script palverify.lua
+
+local machine = manager.machine
+local dbg = machine.debugger
+local screens = machine.screens
+
+local at = tonumber(os.getenv("PAL_FRAME") or "600")
+local out = os.getenv("PAL_OUT") or "/home/arti/code/k-space/vt1200/harness/mame/pal_snap.png"
+
+local screen = nil
+for tag, s in pairs(screens) do screen = s; break end
+if not screen then print("ERROR: no screen device"); machine:exit(); return end
+
+local n = 0
+emu.register_frame_done(function()
+	n = n + 1
+	if n == at then
+		local err = screen:snapshot(out)
+		print(string.format("SNAP frame=%d -> %s err=%s", n, out, tostring(err)))
+		machine:exit()
+	end
+	if n > at + 5 then machine:exit() end
+end, "vt1200-palverify")
+
+if dbg then dbg.execution_state = 'run' end

harness/mame/vt1200.cpp

@@ -1196,7 +1196,10 @@ void vt1200_state::vt1200(machine_config &config)
 
 	// 256 entries so the TMS34010 renderer can encode multi-bit pixels; the
 	// terminal's real output stage is a later question.
-	PALETTE(config, m_palette).set_entries(256);
+	// Explicit monochrome ramp: pen 0 = black, pen 1 = white.  render_active_line()
+	// writes only pens 0/1, so with no init delegate the on-screen colour was
+	// whatever MAME's default palette held (reported: red).  Two entries suffice.
+	PALETTE(config, m_palette, palette_device::MONOCHROME).set_entries(2);
 
 	// The two Signetics SCN2681 DUARTs (same family as the Motorola MC68681:
 	// 2681 = RD/WR strobes, 68681 = R/W + DTACK + interrupt vector).  The