refactor(system-lib): extract cj/file-from-context from system-utils

Phase 2.4 of utility-consolidation, the last item in the spec's recommended order. `cj/--file-from-context' resolves "the current file" via a three-step fallback chain (explicit arg, `buffer-file-name', dired file at point) -- a useful pattern for any command that operates on the current file regardless of which kind of buffer the user is in.  Promote to public `cj/file-from-context' and re-home in system-lib.el so other modules (mail capture, external-open, AI conversation, dirvish helpers) can use it without an awkward dependency on system-utils.

Migrate the two callers in system-utils.el (`cj/open-this-file-with' and `cj/open-file-with-command') and add `(require \='system-lib)' there per the Phase 2 exit criterion.

Move the existing 7-test file to `tests/test-system-lib-file-from-context.el' and update its references to the new public name.  The test shape is unchanged: 4 Normal + 3 Boundary cases covering explicit-arg precedence, buffer-file-name fallback, dired fallback, and the all-nil case.

1 files changed, 76 insertions, 0 deletions

diff --git a/tests/test-system-lib-file-from-context.el b/tests/test-system-lib-file-from-context.el
new file mode 100644
index 00000000..050535c2
--- /dev/null
+++ b/tests/test-system-lib-file-from-context.el
@@ -0,0 +1,76 @@
+;;; test-system-lib-file-from-context.el --- Tests for cj/file-from-context -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/file-from-context' in system-lib.el.  The helper
+;; returns a file path from the current context, resolving in priority
+;; order: explicit argument, `buffer-file-name', dired file at point.
+;; Returns nil when none of these yield a file.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-lib)
+
+(defmacro test-ffc--with-context (buffer-file in-dired dired-file &rest body)
+  "Run BODY with context stubbed for testing `cj/file-from-context'.
+BUFFER-FILE becomes the value of `buffer-file-name'.
+IN-DIRED controls `derived-mode-p' (t to simulate dired-mode).
+DIRED-FILE becomes the return value of `dired-file-name-at-point'."
+  (declare (indent 3))
+  `(let ((buffer-file-name ,buffer-file))
+     (cl-letf (((symbol-function 'derived-mode-p)
+                (lambda (&rest modes)
+                  (and ,in-dired (memq 'dired-mode modes))))
+               ((symbol-function 'dired-file-name-at-point)
+                (lambda () ,dired-file)))
+       ,@body)))
+
+;;; Normal cases
+
+(ert-deftest test-ffc-explicit-wins-over-buffer-file ()
+  "Normal: an explicit filename argument wins over `buffer-file-name'."
+  (test-ffc--with-context "/from-buffer.el" nil nil
+    (should (string= "/explicit.el"
+                     (cj/file-from-context "/explicit.el")))))
+
+(ert-deftest test-ffc-explicit-wins-over-dired ()
+  "Normal: an explicit filename argument wins over dired file at point."
+  (test-ffc--with-context nil t "/from-dired.el"
+    (should (string= "/explicit.el"
+                     (cj/file-from-context "/explicit.el")))))
+
+(ert-deftest test-ffc-buffer-file-used-when-no-explicit ()
+  "Normal: falls back to `buffer-file-name' when no explicit arg."
+  (test-ffc--with-context "/from-buffer.el" nil nil
+    (should (string= "/from-buffer.el"
+                     (cj/file-from-context)))))
+
+(ert-deftest test-ffc-dired-used-when-no-explicit-no-buffer-file ()
+  "Normal: in a dired buffer, falls back to dired file at point."
+  (test-ffc--with-context nil t "/from-dired.el"
+    (should (string= "/from-dired.el"
+                     (cj/file-from-context)))))
+
+;;; Boundary cases
+
+(ert-deftest test-ffc-all-sources-nil-returns-nil ()
+  "Boundary: no explicit, no buffer-file, not in dired -> nil."
+  (test-ffc--with-context nil nil nil
+    (should-not (cj/file-from-context))))
+
+(ert-deftest test-ffc-explicit-nil-uses-fallback-chain ()
+  "Boundary: explicitly passing nil as the arg still uses the fallback chain."
+  (test-ffc--with-context "/from-buffer.el" nil nil
+    (should (string= "/from-buffer.el"
+                     (cj/file-from-context nil)))))
+
+(ert-deftest test-ffc-dired-mode-but-no-file-at-point ()
+  "Boundary: in dired but nothing at point returns nil (buffer-file also nil)."
+  (test-ffc--with-context nil t nil
+    (should-not (cj/file-from-context))))
+
+(provide 'test-system-lib-file-from-context)
+;;; test-system-lib-file-from-context.el ends here