refactor(system-lib): extract cj/shell-quote-argument-readable from dev-fkeys

Phase 2.2 of utility-consolidation. The "quote only when shell-unsafe characters appear, otherwise leave the argument readable" pattern was trapped in dev-fkeys as `cj/--f6-shell-quote-argument' alongside its `cj/--f6-shell-safe-argument-regexp' constant. Lift both into system-lib.el under their generic names; the F6 branding hid that the same shape is useful for any generated compile/test command where the surrounding line ends up in a *compilation* buffer the user reads.

Six Normal/Boundary tests cover safe inputs that pass through unchanged (alphanumeric paths, test regexes, `FLAG=value', `host:port'), unsafe inputs that get quoted (spaces, `$', `;', `&', backticks, `*'), and the empty-string boundary.

Migrate dev-fkeys's five callers to the new name and add `(require \='system-lib)' per the Phase 2 exit criterion.

1 files changed, 18 insertions, 0 deletions

diff --git a/modules/system-lib.el b/modules/system-lib.el
index f932353f..dc1f8316 100644
--- a/modules/system-lib.el
+++ b/modules/system-lib.el
@@ -36,6 +36,24 @@ keep working."
          :warning)
         nil)))
 
+(defconst cj/shell-safe-argument-regexp "\\`[[:alnum:]_./=+@%:,^-]+\\'"
+  "Regexp matching shell arguments safe to interpolate unchanged.
+Members of this character set survive shell parsing without quoting,
+so a command line containing only these characters in each argument
+remains both safe and readable.")
+
+(defun cj/shell-quote-argument-readable (argument)
+  "Quote ARGUMENT for shell command interpolation when needed.
+
+When ARGUMENT consists only of characters in `cj/shell-safe-argument-regexp'
+it is returned unchanged so the surrounding command stays human-readable
+(useful for compile/test command lines you'll inspect in *compilation*).
+Otherwise falls back to `shell-quote-argument' so the result is safe to
+interpolate."
+  (if (string-match-p cj/shell-safe-argument-regexp argument)
+      argument
+    (shell-quote-argument argument)))
+
 (defun cj/log-silently (format-string &rest args)
   "Append formatted message (FORMAT-STRING with ARGS) to *Messages* buffer.
 This does so without echoing in the minibuffer."