refactor: extract wttrin--mode-line-stale-p helper

The staleness check `(> age (* 2 wttrin-mode-line-refresh-interval))` lived in two places, `wttrin--mode-line-tooltip` and `wttrin--mode-line-update-display`, along with the four-line preamble that read the timestamp out of the cache cons and computed age. Centralizing the rule in a single helper means the threshold lives in one spot. That makes it easy to add a `wttrin-mode-line-staleness-threshold` defcustom later if the magic 2× ever needs to be tunable.

The helper takes a cache entry (or nil) and returns t/nil. Five new tests cover Normal (fresh, stale) and Boundary (just below the threshold, just past, nil entry). The boundary tests use 199s and 201s against a 100s refresh interval to lock the strict `>` semantics with comfortable float-time margins.

In `wttrin--mode-line-update-display` the refactor also drops two locals (`timestamp` and `age`) that were no longer used after the helper call replaced the inline calculation. Behavior is unchanged at both call sites.

2 files changed, 49 insertions, 5 deletions

diff --git a/tests/test-wttrin--mode-line-helpers.el b/tests/test-wttrin--mode-line-helpers.el
index 34523a8..6eae822 100644
--- a/tests/test-wttrin--mode-line-helpers.el
+++ b/tests/test-wttrin--mode-line-helpers.el
@@ -135,5 +135,41 @@ distinguish a missing key from a present key bound to nil."
                        'mode-line-highlight)))
     (test-wttrin--mode-line-helpers-teardown)))
 
+;;; --------------------------------------------------------------------------
+;;; wttrin--mode-line-stale-p
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--mode-line-stale-p-normal-fresh-cache-returns-nil ()
+  "Cache entry well within the refresh window is not stale."
+  (let ((wttrin-mode-line-refresh-interval 100))
+    (let ((entry (cons (- (float-time) 50) "data")))
+      (should-not (wttrin--mode-line-stale-p entry)))))
+
+(ert-deftest test-wttrin--mode-line-stale-p-normal-stale-cache-returns-t ()
+  "Cache entry older than 2× refresh-interval is stale."
+  (let ((wttrin-mode-line-refresh-interval 100))
+    (let ((entry (cons (- (float-time) 500) "data")))
+      (should (wttrin--mode-line-stale-p entry)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--mode-line-stale-p-boundary-just-below-threshold ()
+  "Age 1s below the 2× threshold is not stale (strict >)."
+  (let ((wttrin-mode-line-refresh-interval 100))
+    (let ((entry (cons (- (float-time) 199) "data")))
+      (should-not (wttrin--mode-line-stale-p entry)))))
+
+(ert-deftest test-wttrin--mode-line-stale-p-boundary-just-past-threshold ()
+  "Age 1s past the 2× threshold is stale."
+  (let ((wttrin-mode-line-refresh-interval 100))
+    (let ((entry (cons (- (float-time) 201) "data")))
+      (should (wttrin--mode-line-stale-p entry)))))
+
+(ert-deftest test-wttrin--mode-line-stale-p-boundary-nil-cache-returns-nil ()
+  "Nil cache-entry returns nil with no signal."
+  (should-not (wttrin--mode-line-stale-p nil)))
+
 (provide 'test-wttrin--mode-line-helpers)
 ;;; test-wttrin--mode-line-helpers.el ends here
diff --git a/wttrin.el b/wttrin.el
index 9036a5f..eb66868 100644
--- a/wttrin.el
+++ b/wttrin.el
@@ -676,6 +676,16 @@ On failure with no cache, shows error placeholder."
              ;; No cache at all — show error placeholder
              (wttrin--mode-line-update-placeholder-error))))))))
 
+(defun wttrin--mode-line-stale-p (cache-entry)
+  "Return non-nil if CACHE-ENTRY is stale.
+Stale means age greater than 2 × `wttrin-mode-line-refresh-interval'.
+CACHE-ENTRY is a (TIMESTAMP . WEATHER-STRING) cons or nil.  A nil
+entry returns nil so callers can pass `wttrin--mode-line-cache' directly
+without a separate guard."
+  (when cache-entry
+    (let ((age (- (float-time) (car cache-entry))))
+      (> age (* 2 wttrin-mode-line-refresh-interval)))))
+
 (defun wttrin--mode-line-tooltip (&optional _window _object _pos)
   "Compute tooltip text from `wttrin--mode-line-cache'.
 Calculates age at call time so the tooltip is always current.
@@ -686,7 +696,7 @@ Optional arguments are ignored (required by `help-echo' function protocol)."
     (let* ((timestamp (car wttrin--mode-line-cache))
            (weather-string (cdr wttrin--mode-line-cache))
            (age (- (float-time) timestamp))
-           (stale-p (> age (* 2 wttrin-mode-line-refresh-interval)))
+           (stale-p (wttrin--mode-line-stale-p wttrin--mode-line-cache))
            (age-str (wttrin--format-age age)))
       ;; Re-render emoji if staleness state has changed
       (unless (eq stale-p wttrin--mode-line-rendered-stale)
@@ -702,10 +712,8 @@ Reads cached weather data, computes age, and sets the mode-line string.
 If data is stale (age > 2x refresh interval), dims the emoji and
 shows staleness info in tooltip."
   (when wttrin--mode-line-cache
-    (let* ((timestamp (car wttrin--mode-line-cache))
-           (weather-string (cdr wttrin--mode-line-cache))
-           (age (- (float-time) timestamp))
-           (stale-p (> age (* 2 wttrin-mode-line-refresh-interval))))
+    (let* ((weather-string (cdr wttrin--mode-line-cache))
+           (stale-p (wttrin--mode-line-stale-p wttrin--mode-line-cache)))
       (wttrin--debug-log "mode-line-display: Updating from cache, stale=%s" stale-p)
       ;; Response format is "Location: ☀️ +72°F Clear" — grab first char after colon
       (let ((emoji (if (string-match ":\\s-*\\(.\\)" weather-string)