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>

com/Makefile

@@ -59,7 +59,7 @@ DEPFLAGS = -MMD -MP -MF $(basename $(@F)).d
 # parallel (as mac/ and the sks/ components already do) rather than one after
 # another -- the serial links were about a quarter of a clean rebuild.
 all : | mac sks
-	$(MAKE) -j commands
+	$(MAKE) -j$(JOBS) commands
 
 # Separate target to build commands (called after dependencies are ready)
 .PHONY: commands
@@ -80,12 +80,12 @@ $(BINDIR) :
 
 # Build mac/ objects
 mac :
-	$(MAKE) -C $(KM) -j
+	$(MAKE) -C $(KM) -j$(JOBS)
 
 # Build sks/ components (depends on mac)
 sks : | mac
-	$(MAKE) -C $(KS)/kutil -j
-	$(MAKE) -C $(KS)/target -j
+	$(MAKE) -C $(KS)/kutil -j$(JOBS)
+	$(MAKE) -C $(KS)/target -j$(JOBS)
 	$(MAKE) -C $(KS)/document
 
 # Explicit rules for each command - link against shared library, output to bin/