refactor(system-lib): extract cj/executable-find-or-warn from mail-config

Phase 2 of utility-consolidation, first commit per the spec's recommended order. `cj/mail--executable-or-warn' was the right pattern -- check executable-find, return the path, otherwise emit a clear `display-warning' naming the feature -- but it was trapped in mail-config and only mail callers benefited. Lift it into `cj/executable-find-or-warn' in system-lib.el with one new argument: an optional GROUP symbol that flows through to `display-warning' (defaulting to `cj/system-lib') so per-feature warning filters keep working. Mail callers pass `mail-config' explicitly.

Migrate `cj/mail--mbsync-command' and `cj/mail-configure-smtpmail' to the new helper. Drop the local definition. Add `(require \='system-lib)' to mail-config.el per the spec's Phase 2 exit criterion ("consumer modules explicitly require system-lib").

Five Normal/Boundary tests cover the four return-shape cases (program found / program missing / warning content / default vs explicit group).

Other consumers (prog-*.el, dirvish-config.el, browser-config.el) still call `executable-find' directly. Migrating them is a follow-up commit, audited per call site -- the spec flags some `:if' silent checks as intentional and they should NOT switch to the warning helper.

1 files changed, 19 insertions, 0 deletions

diff --git a/modules/system-lib.el b/modules/system-lib.el
index 4c2f17ef..f932353f 100644
--- a/modules/system-lib.el
+++ b/modules/system-lib.el
@@ -17,6 +17,25 @@ PROGRAM should be a string naming an executable program."
        (not (string-empty-p program))
        (executable-find program)))
 
+(defun cj/executable-find-or-warn (program feature &optional group)
+  "Return PROGRAM's executable path, or warn that FEATURE is unavailable.
+
+When PROGRAM is in PATH, return its absolute path.  When it isn't,
+emit a `display-warning' naming PROGRAM and FEATURE so the user gets
+a clear hint about what won't work, and return nil.
+
+GROUP is the symbol passed to `display-warning' for filtering and
+defaults to `cj/system-lib'.  Callers should pass their own module
+symbol (for example `mail-config') so per-feature warning filters
+keep working."
+  (or (executable-find program)
+      (progn
+        (display-warning
+         (or group 'cj/system-lib)
+         (format "%s not found; %s unavailable" program feature)
+         :warning)
+        nil)))
+
 (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."