Verbatim-safe typography, ^-punctuation quoting, full-range ^UUUU^, polyglot html

Typographic transforms (---, quote pairs, ~) no longer touch verbatim
text: @c/@code/@source_listing content and ^'...'^ spans show exactly the
characters written. "^" before any punctuation character quotes it in
every target (the apostrophe excepted: ^' opens a literal span), with the
new :resolve option on @@@target declaring per-target renderings. The
^UUUU^ code-point form accepts 4-6 hex digits, the full Unicode range.
The html output and transform spellings are polyglot (XML-valid), in
preparation for an EPUB target. New suites: transform_test, character_test
(engine), typography_test (SKS).

(from dev 07ce5ea86a0a)

sks/target/latex_util.py

@@ -35,7 +35,7 @@ def block(material):
     required to know that a target distinguishes horizontal from
     vertical mode.  Asserted by sks/tst/paragraph_test.sh.
     """
-    return "\\par\n" + material.strip("\n") + "\n\\par\n"
+    return "^'\\par'^\n" + material.strip("\n") + "\n^'\\par'^\n"
 
 def environment(name, body, required=None, optional=None):
     req = f"{{{required}}}" if required else ""
@@ -164,3 +164,35 @@ def add_caption(element, caption_label, number, caption_text, latex_width,
             element = minipage(caption + "\\rule[-0.75\\baselineskip]{0pt}{0pt}\n" + element,
                                latex_width, vertical="t")
     return caption_wrapper(element, hpos, offset=offset)
+
+def define_color(name, rgb_or_hex):
+    if "," in rgb_or_hex:
+        r, g, b = [float(e) for e in rgb_or_hex.split(",")]
+    else:
+        def c(h):
+            return float(eval('0x' + h)) / 255.0
+        r = c(rgb_or_hex[:2])
+        g = c(rgb_or_hex[2:4])
+        b = c(rgb_or_hex[4:])
+    return r'\\definecolor{{{}}}{{rgb}}{{{:.3f},{:.3f},{:.3f}}}'.format(name, r, g, b)
+
+COLOR_BOX_TEMPLATE = r'''
+^'\hfill\begingroup
+COLOR
+\setlength{\fboxsep}{8pt}
+\fcolorbox{bordercolor}{localcolor}{
+  \parbox{WIDTH\linewidth}{\raggedright\setlength{\parskip}{8pt}'^
+    TEXT
+^'}}\endgroup'^
+'''.strip()
+
+def color_box(text, width=1, box_color="1,1,1", border_color="0,0,0"):
+    color_definitions = define_color("localcolor", box_color) + "\n" \
+        +  define_color("bordercolor", border_color)
+    result = COLOR_BOX_TEMPLATE
+    for old, new in [['TEXT', re.sub(r'\\', r'\\\\', text)],
+                     ['COLOR', color_definitions],
+                     ['WIDTH', str(width)]]:
+        result = re.sub(old, new, result)
+    result = block(result)
+    return result