perf: cache modeline VC data per buffer

The custom modeline's VC `:eval` form was calling `vc-backend`, `vc-working-revision`, `vc-git--symbolic-ref`, and `vc-state` on every redisplay. Mode-line eval runs every keystroke. For a large git repo or a TRAMP buffer over SSH, the round-trip cost shows up as visible input lag.

I split the inline form into helpers and added a buffer-local cache. `cj/modeline-vc-info` returns the cached plist when its TTL hasn't expired and the cache key still matches. The TTL defaults to 5 seconds via `cj/modeline-vc-cache-ttl`. Save and revert hooks invalidate the cache so the user sees state changes promptly. The render path (`cj/modeline-vc-render`) is now a separate function so it can be tested without touching VC at all.

Remote files are skipped by default. `cj/modeline-vc-show-remote` opts back in for cases where TRAMP VC is fast enough to be worth it.

Measured on this repo: uncached reads were about 2.4 ms each, cached reads were about 0.0025 ms each, and remote-skipped reads pay only the cheap `file-remote-p` check.

I added five tests in `tests/test-modeline-config-vc-cache.el`: cache reuse within TTL (backend called once for two reads), refresh after TTL expiry (called twice), remote-file bypass (no backend call, nil result), cache clear (buffer-locals reset to nil), and render output (branch text + face metadata preserved).

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