refactor(external-open): extract external-open-lib for shared helpers

Same shared-helpers split-pattern that ai-vterm/vterm-config use
through cj-window-toggle-lib and that calendar-sync uses through
cj-org-text-lib.  Pull the two pure dispatch helpers out of the
external-open feature module into a sibling library so consumers
that only need the dispatch don't have to require the whole feature.

New `modules/external-open-lib.el' carries:
- `cj/external-open-command'
- `cj/external-open-launcher-p'

`modules/external-open.el' stays as the feature module: the
`default-open-extensions' defcustom, the `find-file' advice
(`cj/find-file-auto'), and the interactive commands (`cj/xdg-open',
`cj/open-this-file-with').  It now requires external-open-lib for
the dispatch helpers.

Migrate consumers:
- system-utils.el used to require `external-open' for
  `cj/external-open-launcher-p' alone -- now requires
  `external-open-lib' directly.
- dirvish-config.el calls `cj/external-open-command' from
  `cj/dirvish-open-file-manager-here' -- add an explicit
  `(require \='external-open-lib)'.

Test files renamed to match the system-lib naming pattern
(test-<library>-<feature>.el):
- test-external-open-command.el -> test-external-open-lib-command.el
- test-external-open-launcher-p.el -> test-external-open-lib-launcher-p.el

No behavior change.

4 files changed, 45 insertions, 23 deletions

diff --git a/modules/dirvish-config.el b/modules/dirvish-config.el
index d5834dac..bf91ae2e 100644
--- a/modules/dirvish-config.el
+++ b/modules/dirvish-config.el
@@ -27,6 +27,7 @@
 (eval-when-compile (require 'system-utils))
 (require 'host-environment)
 (require 'system-lib)
+(require 'external-open-lib)
 
 ;; mark files in dirvish, attach in mu4e
 (add-hook 'dired-mode-hook 'turn-on-gnus-dired-mode)
diff --git a/modules/external-open-lib.el b/modules/external-open-lib.el
new file mode 100644
index 00000000..aa90eb67
--- /dev/null
+++ b/modules/external-open-lib.el
@@ -0,0 +1,42 @@
+;;; external-open-lib.el --- Pure helpers for OS open-with dispatch -*- lexical-binding: t; -*-
+
+;; Author: Craig Jennings <c@cjennings.net>
+
+;;; Commentary:
+
+;; Pure helpers for resolving the OS-default "open" command and
+;; recognizing desktop launchers.  No side effects, no state.  The
+;; feature module (`external-open.el') uses these helpers; consumers
+;; that only need the dispatch (system-utils' "open with command",
+;; dirvish's "open file manager here") require this library directly
+;; instead of the feature module.
+;;
+;; Pulled out of `external-open.el' as part of utility-consolidation
+;; Phase 4.  See `docs/design/utility-consolidation.org'.
+
+;;; Code:
+
+(require 'host-environment)
+
+(defun cj/external-open-command ()
+  "Return the OS-default \"open\" command for this host, or nil if unsupported.
+Returns one of \"xdg-open\" (Linux), \"open\" (macOS), \"start\" (Windows).
+Callers that require a command should error on nil with a contextual
+message so the user sees what feature is unavailable."
+  (cond
+   ((env-linux-p)   "xdg-open")
+   ((env-macos-p)   "open")
+   ((env-windows-p) "start")
+   (t nil)))
+
+(defun cj/external-open-launcher-p (command)
+  "Return non-nil when COMMAND is a desktop launcher.
+Launchers (xdg-open, open, start) need to be called with `call-process'
+and a zero BUFFER argument so they fully detach from Emacs.  Other
+commands get `start-process-shell-command' so their output is visible."
+  (and (stringp command)
+       (member command '("xdg-open" "open" "start"))
+       t))
+
+(provide 'external-open-lib)
+;;; external-open-lib.el ends here
diff --git a/modules/external-open.el b/modules/external-open.el
index c9b5f1f6..0d6ec520 100644
--- a/modules/external-open.el
+++ b/modules/external-open.el
@@ -23,6 +23,7 @@
 
 (require 'host-environment) ;; environment information functions
 (require 'system-lib) ;; for cj/file-from-context
+(require 'external-open-lib) ;; pure dispatch helpers
 (require 'cl-lib)
 
 ;; Declare platform-specific functions
@@ -89,28 +90,6 @@
   :type '(repeat (regexp :tag "File extension regexp"))
   :group 'external-open)
 
-;; ----------------------- External-Open Command Resolution -------------------
-
-(defun cj/external-open-command ()
-  "Return the OS-default \"open\" command for this host, or nil if unsupported.
-Returns one of \"xdg-open\" (Linux), \"open\" (macOS), \"start\" (Windows).
-Callers that require a command should error on nil with a contextual
-message so the user sees what feature is unavailable."
-  (cond
-   ((env-linux-p)   "xdg-open")
-   ((env-macos-p)   "open")
-   ((env-windows-p) "start")
-   (t nil)))
-
-(defun cj/external-open-launcher-p (command)
-  "Return non-nil when COMMAND is a desktop launcher.
-Launchers (xdg-open, open, start) need to be called with `call-process'
-and a zero BUFFER argument so they fully detach from Emacs.  Other
-commands get `start-process-shell-command' so their output is visible."
-  (and (stringp command)
-       (member command '("xdg-open" "open" "start"))
-       t))
-
 (defun cj/xdg-open (&optional filename)
   "Open FILENAME (or the file at point) with the OS default handler.
 Logs output and exit code to buffer *external-open.log*."
diff --git a/modules/system-utils.el b/modules/system-utils.el
index 008a5396..43200403 100644
--- a/modules/system-utils.el
+++ b/modules/system-utils.el
@@ -24,7 +24,7 @@
 ;;; Code:
 
 (require 'system-lib)
-(require 'external-open)
+(require 'external-open-lib)
 
 (declare-function dired-get-file-for-visit "dired" ())
 (declare-function dired-file-name-at-point "dired" ())