Option sets: a .o target for shared parameters

A named group of optional parameters, declared once and used by several
klammers, so a writer learns one vocabulary instead of a spelling per
klammer.  The "o" target is a pseudo-target beside "k": "k" declares a
klammer's interface and documents it, "o" declares an option interface and
documents it, and neither produces output for any target.

    @@caption_args.o :caption :number.bool true :caption_side.side
    : Arguments that define a caption for a block element @@

    @@code.k :filename @hpos_args :hpos left @ @caption_args :caption_side top @
    | text.literal : A source file displayed verbatim @@

A set is used only in the parameter list of a ".k" declaration -- the one
place a klammer's interface is declared once for all of its targets -- and
is resolved as that list is read.  Names and types come from the set; a
default may be overridden where it is used.  A klammer application in a
parameter list is now a definition-time error.

The SKS gains the sets caption_args and hpos_args (:hpos and :offset), and
@table, @image, @image_grid, @reference and @show gain .k declarations.  A
distance is no longer written as a position: :hpos 4em is rejected, and the
same layout is :hpos left :offset 4em.  Code listings are numbered by
default, like tables and figures.

New engine sources mac/option_set{,_registry}.{h,cpp}; tst/ ships two more
suites, option_set_test.sh and signature_test.sh (twelve in all).

 (from dev 34e536cb0329)

sks/block/block.k

@@ -1,3 +1,26 @@
+#[
+A paragraph is a document-structural object, like a section or a table of
+contents, and making paragraphs is a service @document provides: inside a
+@document, blank-line-separated text becomes paragraphs (the LaTeX
+convention) and @par need not be written.  @par is the explicit form, for
+a text FRAGMENT rendered without a @document, where nothing infers them.
+Written inside a @document it is harmless: html sees a <p>, which the
+paragraph pass recognizes as a block and leaves alone, and \par in
+vertical mode is a no-op.
+]#
+
+@@par.k s : A paragraph of text @@
+@@par.html :: <p>*s*</p> @@
+@@par.tex ::
+\par
+*s*
+\par
+@@
+# In plain text a paragraph is delimited by blank lines, which
+# phases.justify_blocks then fills; #/2 inserts them without depending on
+# the definition body's own whitespace surviving extraction.
+@@par.txt :: #/2*s*#/2 @@
+
 @@sp.k : Non-breaking space character @@
 @@sp.html :: &^#160; @@
 @@sp.tex :: ~ @@

sks/block/block.py

@@ -27,7 +27,7 @@ class Indent(klammer_base.Klammer_base):
             result = tab + latex_util.minipage(
                 "\\raggedright " + self.s, f"\\textwidth - {self.w}ex", center=False, vmargin="4pt")
         #print(result)
-        return result
+        return latex_util.block(result)
 
     def txt(self):
         indent = " " * self.w
@@ -54,13 +54,15 @@ class Note(klammer_base.Klammer_base):
             #width = r'\\textwidth - 16pt - {}\\leftmargin'.format(self.level)
             width = r'\\linewidth - \\leftmargin + 2pt'
 
+        # The \par on each side comes from latex_util.block() below: an
+        # \fcolorbox is box material and must sit in vertical mode.
         result = '''
-\\par\\begingroup
+\\begingroup
 COLOR\\setlength{\\fboxsep}{8pt}
   \\fcolorbox{bordercolor}{localcolor}{
    \\parbox{WIDTH}{\\raggedright\\setlength{\\parskip}{8pt}
       \\textbf{LABEL:} TEXT
-}}\\endgroup\\par
+}}\\endgroup
 '''
         result = re.sub('LABEL', self.label, result)
         result = re.sub('TEXT', re.sub(r'\\', r'\\\\', self.s), result)
@@ -68,7 +70,7 @@ COLOR\\setlength{\\fboxsep}{8pt}
         result = re.sub('COLOR', r'\\definecolor{{bordercolor}}{{rgb}}{{{}}}\n'.format(self.bordercolor), result)
         result = re.sub('WIDTH', width, result)
         print(result)
-        return result
+        return latex_util.block(result)
 
 
 class Block(klammer_base.Klammer_base):
@@ -80,6 +82,9 @@ class Block(klammer_base.Klammer_base):
         return ""
 
     def tex(self):
+        # NOT latex_util.block(): a textblock is absolutely positioned and
+        # does not participate in the normal flow, so breaking the paragraph
+        # around it would move the surrounding text.
         to_x, to_y = [float(e) for e in self.to.split()]
         pt_x, pt_y  = [float(e) for e in self.point.split()]
         result = f"""