fix(host-environment): detect battery correctly on Linux desktops

`env-laptop-p` treated any `battery-format "%B"` value that wasn't literally "N/A" as "has a battery." On a Linux desktop using `battery-upower`, the result is "unknown". The AC adapter and USB-C power entries exist in /sys but there's no BAT*. That made desktops look like laptops. The per-machine font height switch in `font-config.el` broke as a result.

The fix uses /sys/class/power_supply/BAT* as the canonical Linux signal. That's what the kernel exposes, and what upower itself reads. Other platforms keep the `battery-format` path, but the fallback now checks for a live battery status char ("!", "+", "-") instead of only excluding "N/A".

Two pure helpers (`env--battery-status-char-indicates-battery-p`, `env--power-supply-has-battery-p`) keep the logic testable. The new test file covers Normal, Boundary, and Error cases for each helper.

2 files changed, 98 insertions, 5 deletions

diff --git a/modules/host-environment.el b/modules/host-environment.el
index af9248c2..832cc195 100644
--- a/modules/host-environment.el
+++ b/modules/host-environment.el
@@ -12,12 +12,37 @@
 
 
 
+(defun env--battery-status-char-indicates-battery-p (status)
+  "Return non-nil if STATUS indicates a real battery is present.
+STATUS is the value returned by `battery-format' with the \"%B\" spec.
+A real battery reports \"!\" (critical), \"+\" (charging), or \"-\"
+(discharging).  Desktops report \"N/A\", \"unknown\", or empty, which
+all mean no battery."
+  (and (stringp status)
+	   (member status '("!" "+" "-"))
+	   t))
+
+(defun env--power-supply-has-battery-p (power-supply-dir)
+  "Return non-nil if POWER-SUPPLY-DIR contains a BAT* entry.
+Canonical Linux check.  A laptop exposes directories like
+/sys/class/power_supply/BAT0; a desktop exposes only AC adapters
+and USB-C power entries."
+  (and (file-directory-p power-supply-dir)
+	   (file-expand-wildcards (expand-file-name "BAT*" power-supply-dir))
+	   t))
+
 (defun env-laptop-p ()
-  "Non-nil if a battery is present."
-  (when (and (require 'battery nil 'noerror)
-			 battery-status-function)
-	(not (string= "N/A"
-				  (battery-format "%B" (funcall battery-status-function))))))
+  "Non-nil if the host has a battery.
+On Linux, checks /sys/class/power_supply for BAT* entries, which is the
+canonical signal.  On other platforms, falls back to `battery-format'
+\"%B\" and checks for a live battery status char."
+  (cond
+   ((eq system-type 'gnu/linux)
+	(env--power-supply-has-battery-p "/sys/class/power_supply"))
+   ((and (require 'battery nil 'noerror)
+		 battery-status-function)
+	(env--battery-status-char-indicates-battery-p
+	 (battery-format "%B" (funcall battery-status-function))))))
 
 (defun env-desktop-p ()
   "Return t if host is a laptop (has a battery), nil if not."
diff --git a/tests/test-host-environment.el b/tests/test-host-environment.el
new file mode 100644
index 00000000..9290a32f
--- /dev/null
+++ b/tests/test-host-environment.el
@@ -0,0 +1,68 @@
+;;; test-host-environment.el --- Tests for host-environment.el -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `env-laptop-p' and its pure helpers.
+;;
+;; Regression driver: upower on a desktop with only an AC adapter reports
+;; `(battery-format "%B" ...)' as "unknown", not "N/A".  The previous
+;; implementation treated any non-"N/A" value as "has a battery", so
+;; desktops were misclassified as laptops.
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'host-environment)
+
+;;; env--battery-status-char-indicates-battery-p
+
+(ert-deftest test-host-environment-battery-char-live-codes ()
+  "Normal: \"+\", \"-\", and \"!\" indicate a live battery."
+  (should (env--battery-status-char-indicates-battery-p "+"))
+  (should (env--battery-status-char-indicates-battery-p "-"))
+  (should (env--battery-status-char-indicates-battery-p "!")))
+
+(ert-deftest test-host-environment-battery-char-na-returns-nil ()
+  "Boundary: legacy \"N/A\" sentinel means no battery."
+  (should-not (env--battery-status-char-indicates-battery-p "N/A")))
+
+(ert-deftest test-host-environment-battery-char-unknown-returns-nil ()
+  "Boundary: upower \"unknown\" (desktop with AC adapter) means no battery."
+  (should-not (env--battery-status-char-indicates-battery-p "unknown")))
+
+(ert-deftest test-host-environment-battery-char-empty-returns-nil ()
+  "Boundary: empty string means no battery."
+  (should-not (env--battery-status-char-indicates-battery-p "")))
+
+(ert-deftest test-host-environment-battery-char-nil-returns-nil ()
+  "Error: nil input means no battery."
+  (should-not (env--battery-status-char-indicates-battery-p nil)))
+
+;;; env--power-supply-has-battery-p
+
+(ert-deftest test-host-environment-power-supply-with-bat-dir ()
+  "Normal: a directory with BAT0 subdirectory reports a battery."
+  (let ((dir (make-temp-file "power-supply-test-" t)))
+    (unwind-protect
+        (progn
+          (make-directory (expand-file-name "BAT0" dir))
+          (should (env--power-supply-has-battery-p dir)))
+      (delete-directory dir t))))
+
+(ert-deftest test-host-environment-power-supply-no-bat-dir ()
+  "Boundary: a directory with only AC adapter entries reports no battery."
+  (let ((dir (make-temp-file "power-supply-test-" t)))
+    (unwind-protect
+        (progn
+          (make-directory (expand-file-name "ACAD" dir))
+          (make-directory (expand-file-name "ucsi-source-psy-USBC000:001" dir))
+          (should-not (env--power-supply-has-battery-p dir)))
+      (delete-directory dir t))))
+
+(ert-deftest test-host-environment-power-supply-missing-dir-returns-nil ()
+  "Error: nonexistent power-supply directory is treated as no battery."
+  (should-not (env--power-supply-has-battery-p "/nonexistent/path/for-test-12345")))
+
+(provide 'test-host-environment)
+;;; test-host-environment.el ends here