feat: unified cache and staleness handling for mode-line and buffer

Replace TTL-based cache invalidation with proactive scheduled refresh.
Both mode-line and buffer systems now follow: timer refreshes cache,
display reads from cache, staleness indicated when data is old.

Phase 1 - Mode-line cache formalization + staleness display:
- Replace wttrin--mode-line-tooltip-data with wttrin--mode-line-cache
  as (timestamp . data) cons cell matching buffer cache pattern
- Add wttrin--format-age helper for human-readable age strings
- Rewrite wttrin--mode-line-update-display to take no arguments,
  read from cache, compute staleness (age > 2x refresh interval),
  dim emoji gray when stale, show staleness info in tooltip
- Rewrite wttrin--mode-line-fetch-weather to write cache on success,
  show stale display on failure with cache, error placeholder without
- Add wttrin--mode-line-update-placeholder-error for first-launch failure

Phase 2 - Remove TTL, add proactive buffer refresh:
- Rename wttrin-cache-ttl to wttrin-refresh-interval (default 3600s)
  with define-obsolete-variable-alias for backward compatibility
- Change wttrin-mode-line-refresh-interval default from 900 to 3600
- Remove TTL check from wttrin--get-cached-or-fetch; serve cached data
  regardless of age, background timer keeps it fresh
- Add buffer refresh timer (wttrin--buffer-cache-refresh)

Phase 3 - Buffer staleness display:
- Add wttrin--format-staleness-header for buffer age display
- Insert staleness line in wttrin--display-weather before instructions

Phase 4 - Cleanup:
- Remove all references to wttrin--mode-line-tooltip-data
- Update README.org cache settings and mode-line documentation
- Update tests for new API (198 tests across 21 files, all passing)

1 files changed, 12 insertions, 3 deletions

diff --git a/tests/testutil-wttrin.el b/tests/testutil-wttrin.el
index 685cc09..42c8c21 100644
--- a/tests/testutil-wttrin.el
+++ b/tests/testutil-wttrin.el
@@ -72,10 +72,10 @@
   `(let ((wttrin-unit-system ,unit-system))
      ,@body))
 
-(defmacro testutil-wttrin-with-cache-ttl (ttl &rest body)
-  "Execute BODY with wttrin-cache-ttl temporarily set to TTL."
+(defmacro testutil-wttrin-with-refresh-interval (interval &rest body)
+  "Execute BODY with wttrin-refresh-interval temporarily set to INTERVAL."
   (declare (indent 1))
-  `(let ((wttrin-cache-ttl ,ttl))
+  `(let ((wttrin-refresh-interval ,interval))
      ,@body))
 
 (defmacro testutil-wttrin-with-cache-max (max-entries &rest body)
@@ -110,6 +110,15 @@
                   (funcall callback nil)))))
      ,@body))
 
+;;; Mode-line Cache Helpers
+
+(defun testutil-wttrin-set-mode-line-cache (data &optional age-seconds)
+  "Set mode-line cache to DATA, optionally aged by AGE-SECONDS."
+  (let ((timestamp (if age-seconds
+                       (- (float-time) age-seconds)
+                     (float-time))))
+    (setq wttrin--mode-line-cache (cons timestamp data))))
+
 ;;; Test Setup and Teardown
 
 (defun testutil-wttrin-setup ()