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>

doc/install/macos_source_install.md

@@ -86,12 +86,12 @@ then the commands:
 
 ```sh
 cd "$KLAMMERTEXT_HOME"
-make -C com -j               # optimized (-O3) by default; libklammertext.so + sks/*.so + bin/{ktext,kdesc,kdiag}
+make -C com               # optimized (-O3) by default; libklammertext.so + sks/*.so + bin/{ktext,kdesc,kdiag}
 ktext -s '@eval 1 + 1 @' -d  # smoke test — prints 2
 ```
 
 (For a debug build with AddressSanitizer, intended for development, prefix
-`DEBUG=1`: `DEBUG=1 make -C com -j`.)
+`DEBUG=1`: `DEBUG=1 make -C com`.)
 
 For a document smoke test, create a small file and render it to HTML:
 
@@ -208,7 +208,7 @@ To update an existing source installation to the latest version:
 ```sh
 cd "$KLAMMERTEXT_HOME"
 git pull
-make -C com -j               # rebuild library, SKS components, and commands (optimized)
+make -C com               # rebuild library, SKS components, and commands (optimized)
 ```
 
 Rebuild the TeX Live tree only if the SKS's package requirements changed (rare);