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/linux_source_install.md

@@ -106,11 +106,11 @@ first, then the commands. The build is optimized (`-O3`) by default — the buil
 you want to install and run:
 
 ```bash
-make -C com -j        # lib/libklammertext.so + sks/*.so + bin/{ktext,kdesc,kdiag}
+make -C com        # lib/libklammertext.so + sks/*.so + bin/{ktext,kdesc,kdiag}
 ```
 
 (For a slower `-O0` debug build with AddressSanitizer, intended for development,
-prefix `DEBUG=1`: `DEBUG=1 make -C com -j`.)
+prefix `DEBUG=1`: `DEBUG=1 make -C com`.)
 
 Verify the build:
 
@@ -320,7 +320,7 @@ To update an existing source installation to the latest version:
 ```bash
 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)
 ```
 
 The TeX Live tree only needs rebuilding if the SKS's package requirements