Images in plain text and the terminal; bounded build parallelism (from dev 16f82cc887af)

- @image under txt and ansi is a picture in characters, in five styles
  (Image_txt_style: halfblock the default, shade, ascii, quadrant,
  braille), sized to the terminal's width less a margin when ktext runs
  in one, every cell in its own colours under ansi; a figure's caption
  is centred under it and no txt caption is ruled off any more.
- The build compiles as many files at once as the machine has cores
  (JOBS; make -C com JOBS=1 for a small machine): the sub-makes' bare -j
  meant unlimited parallelism and exhausted 8 GB machines. The three
  install pages say "make -C com".

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

sks/image/image.k

@@ -9,6 +9,22 @@
 @@@
 
 @@@state Image_output_dir :desc Output directory for images :value img @@@
+@@@state Image_txt_style
+         :desc How an image is drawn in characters under txt and ansi:
+               halfblock (two pixels per cell in full colour under ansi -- the
+               nearest to the raster, and the default; under txt a solid
+               block, since its picture is only its colours), shade (block
+               shades, coloured tiles under ansi), ascii (a density ramp of
+               typewriter characters), quadrant (four sub-pixels per cell, two
+               colours per cell under ansi), braille (eight dots per cell).
+         :value halfblock
+@@@
+@@@state Image_txt_dark_background
+         :desc Under txt and ansi, the density ramp assumes a dark terminal
+               background (true): bright pixels are the dense characters.
+               false reverses it for a light background.
+         :value true
+@@@
 
 @@@state Image_default_width :desc Default width for images :value .667w @@@
 
@@ -45,10 +61,18 @@
 An image read from a file.  The basename names the file, which is looked
 for in the image search path and converted to a format the target can use.
 ^:width is the image's width in the text column, ^:hpos and ^:offset place
-it there, and the caption arguments caption it.
+it there, and the caption arguments caption it.  Under txt and ansi the
+image is drawn in characters (the Image_txt_style state variable:
+halfblock, the default, or shade, ascii, quadrant, braille), and ansi
+colours them; the
+measure is the terminal's width
+less a small margin when ktext runs in one, else the text width, and
+^:width applies to that (a fraction, a number of columns, or f for all of
+it).  The Image_txt_dark_background state variable (true) reverses the
+ramp for a light terminal.
 @@
 
-@@image.html,tex :: @eval image.Image(K) eval@ @@
+@@image.html,tex,txt :: @eval image.Image(K) eval@ @@
 
 @@image_grid.k
   image_specs.rest(2)