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)

mac/locator.h

@@ -24,6 +24,14 @@ public:
         , m_chr(int(location.column()))
         {};
     Locator(const fs::path& filename, int line, int chr);
+    // "No location": for errors with no document position -- command-line
+    // argument errors, lookups made on the command's behalf.  NOT the same
+    // as Locator(): the default constructor captures the C++ CALL SITE via
+    // std::source_location (and a defaulted Locator parameter captures the
+    // CALLER), which is what leaked "argv.cpp, line 597" into user-facing
+    // errors (found by the error gallery, 2026-08-22).  Locator() is for
+    // logging; errors either carry a real document location or this.
+    static Locator none() { return Locator(fs::path{}, -1, -1); }
     std::string str(bool relative = false) const;
     std::string desc(bool relative = false) const;
     std::string abbrev(bool include_chr=true) const;