perf(tui): async file loading, LRU rendering cache, and bounded variant caps for full-file view

Fixes #833: Decouple synchronous file reading and Chroma highlighting
from View() render loop into an async cache keyed by target path, size,
modtime and diff fingerprint.

- Bound total source bytes consumed with io.LimitReader and immediate
  cutoff on oversized physical lines (> fileViewMaxBytes).
- Bound rendered ANSI variants per cache entry with a 4-slot LRU to prevent
  memory growth across window resizes or changed line mutations.
- Bound memory with 4000 lines / 1 MiB total / 4 KiB line limits and
  evict cleanly on theme changes.

Author: hazyhaar

internal/tui/export_test.go

@@ -144,6 +144,15 @@ func (c *staticRenderCache) stats() renderCacheStats {
 	return c.statsData
 }
 
+func fileViewCacheStatsForTest() fileViewCacheStats {
+	return defaultFileViewCache.stats()
+}
+
+func resetFileViewCacheForTest() {
+	defaultFileViewCache.clear()
+	defaultFileViewCache.resetStats()
+}
+
 func renderSelectableList(options selectableListOptions) string {
 	if len(options.Items) == 0 {
 		return ""