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/code/css/code.css

@@ -2,12 +2,36 @@
     font-family: var(--monospace);
 }
 
+/* A whole listing, inside its :hpos position container.  Full width by
+   default so an annotated listing's comment column has a remainder to take
+   (.code_comment is flex: 1); an UNANNOTATED listing is shrink-to-fit
+   instead, which is what lets :hpos center or right-align it -- a
+   full-width child cannot be moved within its container.  The LaTeX
+   counterpart is Code.tex_place. */
+.code_listing {
+}
+
+.code_listing_box {
+    width: fit-content;
+}
+
+/* An annotated code listing (sks/code/code_block.py).  The same model as
+   the LaTeX side in sty/code.sty: each BLOCK is a run of code lines with
+   an optional comment beside it, the shaded box is as wide as that block's
+   longest line, and comments sit a fixed distance from their own block
+   without aligning with each other.
+
+   align-items: center is the counterpart of LaTeX's \parbox[c] pair.  No
+   vertical margin or padding anywhere in the stack: consecutive blocks
+   must abut exactly, so that adjacent shaded blocks read as one region and
+   the line rhythm is the same whether a line is shaded or not.  That is
+   what \strut and \offinterlineskip buy on the LaTeX side; here it is just
+   the absence of vertical space. */
 .code_block {
     display: flex;
     align-items: center;
-    margin: .125rem 0 0 1rem;
+    margin: 0 0 0 1rem;
     padding: 0;
-/*    flex-direction: column-reverse; */
 }
 
 .code_caption {
@@ -15,20 +39,28 @@
     font-style: italic;
 }
 
+/* white-space: pre keeps the code's own spacing, and the shrink-to-fit
+   width of an inline-block IS the block's longest line — so the shading is
+   one solid rectangle with no per-line work and nothing measured.  Padding
+   is horizontal only; see the note on .code_block. */
 .code_text {
     display: inline-block;
-/*    vertical-align: top; */
     white-space: pre;
     font-family: var(--monospace);
-    line-height: 1.2;
+    line-height: 1.4;
+    /* No horizontal padding: the box's left edge sits ON the code column,
+       where the line's own indentation begins, rather than out to the left
+       of it.  LaTeX counterpart: \klcodepad 0pt. */
+    padding: 0;
 }
 
+/* The comment takes what the block leaves of the containing width — the
+   browser's counterpart of \dimexpr\linewidth-\klcodewidth-\klcodegap.
+   min-width is the counterpart of \klcodemin: below it the comment would
+   be too narrow to set prose in. */
 .code_comment {
-    display: inline-block;
-/*    vertical-align: top; */
-    padding: .25rem;
-    border: solid white 1px;
-    padding: .25rem .25rem .25rem .5rem;
+    flex: 1;
+    padding-left: .75rem;
     font-family: var(--sans-serif);
     font-style: italic;
     font-size: .8rem;
@@ -36,22 +68,23 @@
     line-height: 1.25;
 }
 
+/* An annotated block: background only, no border.  A border would need a
+   matching one on the unshaded blocks to keep the code aligned, and the
+   design settled on shading alone. */
 .code_border {
-    border: solid gray 1px;
-    /* margin: .125rem; */
-    margin: .125rem .125rem .125rem .25rem;
-    padding: .125rem .5rem .25rem .5rem;
-    background-color: rgb(95%,95%,95%);
-
+    background-color: rgb(100%,100%,60%);
+    /* Clear space above and below a boxed block, so two boxed blocks with
+       no unboxed lines between them read as two boxes rather than one.
+       On the block, never on the line: the lines within a block must still
+       abut, or the shading stops being a solid rectangle.  The LaTeX
+       counterpart is \klboxgap in sty/code.sty. */
+    margin: 2px 0;
+    /* The box's own vertical margin: space inside the shading, above the
+       first line and below the last.  LaTeX counterpart: \klboxpad. */
+    padding-top: 2px;
+    padding-bottom: 2px;
 }
 
 .code_no_border {
-    padding: .125rem 0 .125rem .5rem;
-    margin: 0 0 0 .125rem;
-    border: solid white 1px;
-    /* Debugging:
-    border: solid lightgray 1px;
-    background-color: rgb(250,250,127);
-    */
 }