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, 4 insertions, 4 deletions

diff --git a/tests/test-integration-debug.el b/tests/test-integration-debug.el
index db2706b..d4abd9b 100644
--- a/tests/test-integration-debug.el
+++ b/tests/test-integration-debug.el
@@ -45,7 +45,7 @@
   (wttrin-clear-cache)
   (wttrin-debug-clear-log)
   (setq wttrin-mode-line-string nil)
-  (setq wttrin--mode-line-tooltip-data nil))
+  (setq wttrin--mode-line-cache nil))
 
 ;;; Mock URL Fetching
 
@@ -104,9 +104,9 @@ and emoji-extraction events to the debug log."
        (should (stringp wttrin-mode-line-string))
        (should (string-match-p "☀" wttrin-mode-line-string))  ; Should contain emoji
 
-       ;; Verify tooltip data was set
-       (should wttrin--mode-line-tooltip-data)
-       (should (string= test-wttrin-sample-weather-data wttrin--mode-line-tooltip-data))
+       ;; Verify cache was populated with weather data
+       (should wttrin--mode-line-cache)
+       (should (string= test-wttrin-sample-weather-data (cdr wttrin--mode-line-cache)))
 
        ;; Verify debug log captured key events
        (let ((log-messages (mapcar #'cdr wttrin--debug-log)))