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>

mac/Makefile

@@ -76,7 +76,7 @@ ifneq ($(filter clang,$(MAKECMDGOALS)),)
 	@:
 else
 	$(MAKE) clean
-	$(MAKE) -j all
+	$(MAKE) -j$(JOBS) all
 endif
 
 # "make clang" = incremental build; "make clang redo" = full clean rebuild