Escaping, table layout, and document fixes

Quoted Klammertext specials (^@ ^| ^# ^^ ^: ^*) and ^'...'^ regions now
survive re-processing (held as escape markers until final output);
:after_apply phase functions receive and return raw target text.

Tables: :hpos element position (center|left|right|<length>) replaces the
unimplemented :center/:indent; the ranged cell override is renamed
:justify; :column_width works in html (colgroup widths) and gains
'fill' -- the remaining width, capped at the column's widest entry, in
both targets; a table wider than the text column warns on the console;
table edges without an outer line set their text flush on the margins.

@document: no empty title bar for untitled documents; @vfill fills to
the bottom of the window in html (pure CSS); @vspace in plain text;
new @dot klammer; monospace email links.

sks/block/css/block.css

@@ -24,3 +24,28 @@ p {
 .indent {
     margin-left: 2rem;
 }
+
+/* @vfill: LaTeX's \vfill glue as flex-grow.  The :has() rule turns the
+   text column into a flex column ONLY in documents that use @vfill (flex
+   containers do not collapse vertical margins, so paragraph spacing
+   shifts slightly there); several vfills share the free space equally,
+   like \vfill.  The min-height ties the fill to the window: with a title
+   or status bar the column overshoots by their height (a small scroll);
+   content taller than the window collapses the glue, as on a full LaTeX
+   page.  Fills the window, not a browser-printed page. */
+#text:has(.vfill) {
+    display: flex;
+    flex-direction: column;
+    min-height: calc(100vh - 1lh);
+}
+
+/* One line of whitespace below the filled content, so the last block
+   does not touch the bottom of the window.  A margin on the last flex
+   item, not #text padding: #middle's overflow clips the padding. */
+#text:has(.vfill) > :last-child {
+    margin-bottom: 1lh;
+}
+
+.vfill {
+    flex-grow: 1;
+}