Palette slots

SLOTS (pxart help slots)
  A game that recolors a sprite at runtime (each villager's skin, hair, shirt) picks each
  key's color from a list. A palette line lists them after the key's color, split by '|':
    f #e0ac69 | #f5cfa0 | #c68642 | #8d5524 | #ffdbac
  f is a slot: #e0ac69, the first, is its base color, the one every command draws, checks
  and exports (render, anim, compose, export's PNGs, Aseprite and Tiled sheets), as if the
  line ended there; the others are its alternatives. A file with no '|' reads and writes as
  it always did.
  - Alternatives are colors like any key's (#rrggbb, #rrggbbaa, transparent); a bad one is
    E_BAD_COLOR. check --palette P fails an alternative P hasn't got, and notes a color a
    slot lists twice. palette lists them with the key's color; palette --add
    'f=#e0ac69|#f5cfa0' adds a slot, or sets the alternatives of f if it has that color
    (an imported f gets a line of FILE's own, and says how to set it in the palette file).
  - The alternatives are part of the key's color. A key another file has in the same colors
    is the same key; a plain key and a slot of one base color, or two slots with other
    alternatives, are two colors: compose, paste and frames --copy-to call it E_KEY_CONFLICT
    (--rekey gives the incoming key a free one), and diff says '3 px differ only in slot
    alternatives'. A PNG has no alternatives: from-png draws a slot's base color with its
    key.
  - A palette file's slots are imported with its keys. A local line for the key replaces
    the imported one whole (check notes the override). A plain local line of the import's
    base color only hides its alternatives: check says so, and palette --import and --remove
    drop it.
  - A @variant line for a slot is one color, the key's color in the variant whatever the
    slot, or a list as long as the slot's, one color per slot color in order:
      @variant night
      f #6f5634 | #7a6750 | #634321 | #462a12 | #7f6d56
    Another length is E_BAD_COLOR. palette --variant night --derive-from base derives
    each color of a slot, a list. Keys a variant leaves keep their slots.
  - Commands that copy or rename keys (compose, paste, put, frames --copy-to, extract,
    recolor 'a>b', palette --hoist, --extract-to, --import) carry the alternatives with the
    key; recolor f=#hex changes the base color and keeps them.

  render|sheet FILE... --slots N|K=I,...
      Draws the frames once per combination of their file's slots: --slots 6 is the base
      colors, then 5 others, drawn from a fixed seed (the same every run; --slots 8 starts
      with --slots 6's), or every combination when there are no more than N. K=I pins slot
      K to its I-th color (0 is the base), K=#rrggbb to that color: --slots f=2,h=0 draws
      that one, the other slots at base; --slots 6,f=2 samples the others. A pin a file's
      slot can't take is E_BAD_ARG naming the file. Each label ends with its combination
      (walk/0 h3f2c4p1), and a line per combination prints the colors it draws (in
      --variant V's colors: a note names the slots V gives one color) and the --slots that
      draws it alone. With --rows group each combination of an animation is its own row.

  export FILE|DIR[:SEL]... --indexed X.json
      The frames as key grids, with the palette that colors them, for a game to recolor:
      {"version": 1,
       "palette": {"k": "#1a1a1a", "f": "#e0ac69", ...},   every key's base color
       "slots": {"f": ["#e0ac69", "#f5cfa0", ...], ...},   each slot's colors, base first
       "variants": {"night": {"k": "#000000",              the keys each variant sets:
                              "f": ["#6f5634", ...]}},     a slot's list, or one color
       "frames": [{"id": "walk/0", "w": 8, "h": 12, "ms": 125, "pivot": {"x": 4, "y": 11},
                   "rows": ["..hhhh..", ...]}, ...],
       "animations": [{"name": "walk", "from": 0, "to": 3, "direction": "forward",
                       "repeat": 0}, ...]}
      rows: a string per row, a key per pixel, '.' transparent. Colors are #rrggbb, or
      #rrggbbaa when not opaque. A game draws key k in variant V (slot color i) as V's k
      (its i-th when a list), else as slots' k's i-th, else as palette's k. "slots",
      "variants" and "animations" are there when empty too; "pivot" is left out for a
      frame without one. Frames are in export's id order (help CONVERTING), which "from"
      and "to" index; a @still group is no animation. repeat 0 is forever. --prefix-file
      ids frames and names animations FILE/ID. Every file shares one palette and one set of
      variants: a key two files color differently, in the base or in a variant (a file
      without that variant draws its base colors there), is E_KEY_CONFLICT. --indexed goes
      alone or with --frames, --aseprite and --tiled; --variant V colors only their PNGs.