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.

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