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/target/txt_util.py

@@ -58,7 +58,7 @@ def block_width(text_width, indent):
     return max(1, int(text_width) - indent)
 
 def add_caption(element, caption_label, number, caption_text,
-                side="top", indent=0):
+                side="top", indent=0, centered=False):
     """Attach a caption to an already-laid-out block.
 
     `element` is the block's text, indented by `indent` columns of NBSP.
@@ -95,10 +95,14 @@ def add_caption(element, caption_label, number, caption_text,
     # marker substitution, so a caption that arrived there as plain text
     # got none of it: ":number false" silently lost its rule.  The label
     # is what says "number me", not the presence of the marker.
+    # `centered` rides in the marker's name as a "+center" flag, which the
+    # numbering pass reads off: a figure's caption is centred under it, as
+    # in html and tex; a table's or a listing's stays at the block's edge.
+    flag = "+center" if centered else ""
     if number:
-        caption = kutil.caption_marker(caption_label, caption_text, None)
+        caption = kutil.caption_marker(caption_label + flag, caption_text, None)
     elif caption_text:
-        caption = kutil.caption_marker("", caption_text, None)
+        caption = kutil.caption_marker(flag, caption_text, None)
     else:
         return element
     # Marked laid-out, like the block: with no indent the caption begins