Promote Xevious (U) and Zanac (U) from _wip/ to NROM/

- Xevious (U): Promoted from _wip/NROM/Xevious (U)/ to NROM/Xevious (U)/
  - Pass 3 complete: 34.5% code coverage, 143 semantic entry points,
    4 named data tables, ~20 semantic ZP aliases
  - NROM mapper 0, 32 KB PRG, 8 KB CHR, horizontal mirroring

- Zanac (U): Promoted from _wip/UNROM/Zanac (U)/ to NROM/Zanac (U)/
  - Pass 3 complete: 34.5% code coverage (bank 7 only), 133 semantic
    entry points, 26 named data tables, 14 semantic ZP aliases
  - UNROM mapper 2, 128 KB PRG (8 banks), no CHR
  - Bank 7 fixed at CPU -; banks 0-6 switchable at 000-
  - Bank 7 assembled separately as prg.07.s with .org

Both projects build byte-identical. Updated root README.md inventory,
per-project details, and WIP.md accordingly.

Move Zanac (U) to UNROM/ mapper directory

Zanac (U) is a UNROM (mapper 2) game, not NROM (mapper 0). Move it to
the correct mapper directory and update README.md and WIP.md references
accordingly. Update project counts in WIP.md (857->856 total, UNROM
98->97 projects).

Promote Mermaids of Atlantis (U) from _wip/ to MAPPER8/

Additional cleanup of Mermaids of Atlantis

MAPPER8/Mermaids of Atlantis (U)/symbols.inc

@@ -0,0 +1,108 @@
+; =============================================================================
+; Nintendo Entertainment System hardware symbols
+; =============================================================================
+
+; -----------------------------------------------------------------------------
+; PPU registers
+; -----------------------------------------------------------------------------
+
+PPUCTRL             = $2000
+PPUMASK             = $2001
+PPUSTATUS           = $2002
+OAMADDR             = $2003
+OAMDATA             = $2004
+PPUSCROLL           = $2005
+PPUADDR             = $2006
+PPUDATA             = $2007
+
+OAMDMA              = $4014
+
+; -----------------------------------------------------------------------------
+; Controller ports
+;
+; $4016:
+;   Write: controller strobe
+;   Read:  controller port 1
+;
+; $4017:
+;   Read:  controller port 2
+;   Write: APU frame counter
+; -----------------------------------------------------------------------------
+
+JOY1                = $4016
+JOY2                = $4017
+
+; -----------------------------------------------------------------------------
+; APU pulse channel 1
+; -----------------------------------------------------------------------------
+
+APUPULSE1CTRL       = $4000
+APUPULSE1SWEEP      = $4001
+APUPULSE1TIMERLO    = $4002
+APUPULSE1TIMERHI    = $4003
+
+; -----------------------------------------------------------------------------
+; APU pulse channel 2
+; -----------------------------------------------------------------------------
+
+APUPULSE2CTRL       = $4004
+APUPULSE2SWEEP      = $4005
+APUPULSE2TIMERLO    = $4006
+APUPULSE2TIMERHI    = $4007
+
+; -----------------------------------------------------------------------------
+; APU triangle channel
+; -----------------------------------------------------------------------------
+
+APUTRIANGLECTRL     = $4008
+APUTRIANGLETIMERLO  = $400A
+APUTRIANGLETIMERHI  = $400B
+
+; -----------------------------------------------------------------------------
+; APU noise channel
+; -----------------------------------------------------------------------------
+
+APUNOISECTRL        = $400C
+APUNOISEPERIOD      = $400E
+APUNOISELENGTH      = $400F
+
+; -----------------------------------------------------------------------------
+; APU DMC channel
+; -----------------------------------------------------------------------------
+
+APUDMCCTRL          = $4010
+APUDMCDIRECT        = $4011
+APUDMCADDR          = $4012
+APUDMCLENGTH        = $4013
+
+; -----------------------------------------------------------------------------
+; APU status and frame counter
+;
+; $4015:
+;   Write: enable or disable the five APU channels
+;   Read:  channel and interrupt status
+;
+; $4017:
+;   Write: APU frame-counter mode and IRQ inhibit
+;   Read:  controller port 2
+; -----------------------------------------------------------------------------
+
+APUSTATUS           = $4015
+APUFRAMECOUNTER     = $4017
+
+; -----------------------------------------------------------------------------
+; Controller masks
+; -----------------------------------------------------------------------------
+
+PAD_RIGHT           = $01
+PAD_LEFT            = $02
+PAD_DOWN            = $04
+PAD_UP              = $08
+PAD_START           = $10
+PAD_SELECT          = $20
+PAD_B               = $40
+PAD_A               = $80
+
+PAD_HORIZONTAL      = PAD_LEFT | PAD_RIGHT
+PAD_VERTICAL        = PAD_UP | PAD_DOWN
+PAD_DPAD            = PAD_HORIZONTAL | PAD_VERTICAL