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)

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