FANCO v6 experimental RGBA palette and RGB effects
================================================

Only v6 implements these ports. 0070–0078 were scratch bytes in earlier
prototypes and are now intentionally assigned to graphics. There is no
frame-sized RGB RAM allocation in the guest's 64 KiB address space.

Ports (hex addresses, byte accesses unless stated)
------------------------------------------------
0070  Palette write index (0–255)
0071  Staged red (0–255)
0072  Staged green (0–255)
0073  Blue: writing commits staged R,G, this B and staged alpha (0078) to
      the selected RGBA8888 palette entry, then increments 0070 modulo 256.
      Reads return the last written B.
0074  Drawing mode: 0 indexed replace, 1 RGB alpha-over, 2 RGB additive.
      Other values currently behave as indexed replace.
0075  Drawing alpha, 0–255; reset value 255.
0076  Persistence multiplier, 0–255; reset value 255.
0077  Command, write-only action; reads return zero after an action:
      1 clear the indexed framebuffer to index 0 and discard RGB overrides;
      2 materialize the currently visible image into the RGB surface;
      3 materialize and multiply RGB channels by persistence/255.
      Other command values do nothing. Each command 1–3 costs 65536 virtual
      work cycles, in addition to instruction bytes. There is no hidden
      per-presentation decay; the guest issues command 3 once per desired frame.
0078  Staged palette alpha, 0–255; reset value 255. Latched when writing B
      at 0073, NOT when selecting an index. Existing RGB-only uploads therefore
      remain opaque without any new initialization.

Default palette is the old RGB332 conversion, including rounding: red and
green are round(level*255/7), blue is level*85. Palette 0 is initially black.
Every palette entry's alpha resets to 255. Indexed replacement ignores palette
alpha and remains opaque; alpha is applied only in drawing modes 1 and 2.
The palette is RGB888 in the display's ordinary sRGB encoding, NOT linear
light or OKLCH. Alpha is an effect amount, not a transparent canvas output;
the browser receives opaque RGBA. Wide-gamut/OKLCH support is not implemented.

Palette upload example (one entry, then auto-advance):
    mov.b [0x70],32
    mov.b [0x71],20
    mov.b [0x72],100
    mov.b [0x78],128      ; optional per-entry alpha, otherwise stays 255
    mov.b [0x73],240

Temporal and readback semantics
-------------------------------
The indexed framebuffer is retained exactly: OUT always stores its source
index; IN always reads that index, including after blended drawing. No
RGB-to-index quantization or index arithmetic is used for blending.

Every pixel is either indexed or an RGB override. Mode 0 writes an index
and removes that pixel's RGB override. All indexed pixels resolve through
the CURRENT palette, so changing a palette entry recolors previously drawn
indexed pixels without redrawing them. Changing the palette does not
retroactively recolor RGB overrides: these are accumulated historical RGB.

Modes 1 and 2 read the currently visible destination RGB, resolve the source
index through the palette at instruction retirement, then store an RGB
override. Effective alpha A = floor((global_alpha*palette_alpha + 127)/255).
Thus different indices provide different per-pixel alpha in a single BLIT or
OUT stream without changing the global alpha per pixel. With source channel S
and destination channel D, per channel:
    alpha-over: floor((S*A + D*(255-A) + 127)/255)
    additive:   min(255, D + floor((S*A + 127)/255))
Alpha zero still writes the IN-visible index and materializes the unchanged
visible destination. Command 3 uses floor((channel*persistence+127)/255).
Repeated fading can settle on a small nonzero value because of rounding;
command 1 guarantees a complete clear. Palette edits after command 2/3 do
not recolor the materialized image until new indexed replacement writes.

The draw mode and alpha apply equally to OUT and every BLIT destination
pixel. Existing BLIT fill, RAM copy, keyed RAM copy and indexed framebuffer
copy modes retain their clipping, index keying and source-snapshot rules.
Framebuffer BLIT mode 3 copies INDICES, not RGB overrides, and blends those
indices using the current palette and draw controls. This is deliberate
IN-compatible behavior, not a hidden RGB copy operation.

BLIT's alpha-black fill is useful for fading only a rectangle while keeping
another rectangle palette-indexed. Palette index 0 must be black for that.

All effects happen at guest instruction retirement, not in host animation
callbacks. Word stores commit low byte then high byte inside one atomic
instruction; e.g. a word store to 0072 stages G then commits its B high byte.
Palette programming has no extra work charge; it changes one palette entry.
SYS PRESENT captures the visible palette/RGB result before the guest runs
again. Rendering or taking extra host snapshots never changes VM state.

The host performs only palette expansion and presentation. All geometry,
particle motion, palette selection and effect commands belong to guest code.
Loading a new application resets palette RGB/alpha, staged alpha, draw mode,
global alpha and persistence;
ordinary application RAM segments may then initialize the control bytes.
Palette commits themselves require guest writes to 0073.