calc: Allow strings with character codes above Latin-1

The current behavior of the functions 'calc-display-strings',
'strings', and 'bstrings' is to skip any vector containing
integers outside the Latin-1 range (0x00-0xFF).  We introduce a
custom variable 'calc-string-maximum-character' to replace this
hard-coded maximum, and to allow vectors containing higher
character codes to be displayed as strings.  The default value
of 0xFF preserves the existing behavior.

* lisp/calc/calc.el (calc-string-maximum-character): Add custom
variable 'calc-string-maximum-character'.
* lisp/calc/calccomp.el (math-vector-is-string): Replace hard-coded
maximum with 'calc-string-maximum-character', and the 'natnump'
assertion with 'characterp'.  The latter guards against the
maximum being larger than '(max-char)', but not on invalid types of
the maximum such as strings.

* test/lisp/calc/calc-tests.el (calc-math-vector-is-string): Add
tests for 'math-vector-is-string' using different values of
'calc-string-maximum-character'.

* doc/misc/calc.texi (Quick Calculator, Strings, Customizing Calc):
Add variable definition for 'calc-string-maximum-character' and
reference thereof when discussing 'calc-display-strings'.
Generalize a comment about string display and availability of 8-bit
fonts.
(Bug#78528)

lisp/calc/calc.el

@@ -628,6 +628,37 @@ Otherwise, 1 / 0 is changed to uinf (undirected infinity).")
 (defcalcmodevar calc-display-strings nil
   "If non-nil, display vectors of byte-sized integers as strings.")
 
+(defcustom calc-string-maximum-character #xFF
+  "Maximum value of vector contents to be displayed as a string.
+
+If a vector consists of characters up to this maximum value, the
+function `calc-display-strings' will toggle displaying the vector as a
+string.  This maximum value must represent a character (see `characterp').
+Some natural choices (and their resulting ranges) are:
+
+- `0x7F'     (`ASCII'),
+- `0xFF'     (`Latin-1', the default),
+- `0x10FFFF' (`Unicode'),
+- `0x3FFFFF' (`Emacs').
+
+Characters for low control codes are either caret or backslash escaped,
+while others without a glyph are displayed in backslash-octal notation.
+The display of strings containing higher character codes will depend on
+your display settings and system font coverage.
+
+See the following for further information:
+
+- info node `(calc)Strings',
+- info node `(elisp)Text Representations',
+- info node `(emacs)Text Display'."
+  :version "31.1"
+  :type '(choice (restricted-sexp :tag "Character Code"
+                                  :match-alternatives (characterp))
+                 (const :tag "ASCII"   #x7F)
+                 (const :tag "Latin-1" #xFF)
+                 (const :tag "Unicode" #x10FFFF)
+                 (const :tag "Emacs"   #x3FFFFF)))
+
 (defcalcmodevar calc-matrix-just 'center
   "If nil, vector elements are left-justified.
 If `right', vector elements are right-justified.

lisp/calc/calccomp.el

@@ -907,13 +907,20 @@
 		(concat " " math-comp-right-bracket)))))
 
 (defun math-vector-is-string (a)
+  "Return t if A can be displayed as a string, and nil otherwise.
+
+Elements of A must either be a character (see `characterp') or a complex
+number with only a real character part, each with a value less than or
+equal to the custom variable `calc-string-maximum-character'."
   (while (and (setq a (cdr a))
-	      (or (and (natnump (car a))
-		       (<= (car a) 255))
+	      (or (and (characterp (car a))
+		       (<= (car a)
+			   calc-string-maximum-character))
 		  (and (eq (car-safe (car a)) 'cplx)
-		       (natnump (nth 1 (car a)))
+		       (characterp (nth 1 (car a)))
 		       (eq (nth 2 (car a)) 0)
-		       (<= (nth 1 (car a)) 255)))))
+		       (<= (nth 1 (car a))
+			   calc-string-maximum-character)))))
   (null a))
 
 (defconst math-vector-to-string-chars '( ( ?\" . "\\\"" )