docs: refresh immersion and stats documentation

changes/2026-03-16-character-dictionary-startup-dedupe.md

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+type: fixed
+area: anki
+
+- Fixed repeated character-dictionary startup work by scheduling auto-sync only from mpv media-path changes instead of also re-triggering it from connection and media-title events for the same title.

changes/2026-03-16-macos-fullscreen-overlay-stability.md

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+type: fixed
+area: overlay
+
+- Fixed macOS fullscreen overlay stability by keeping the passive visible overlay from stealing focus, re-raising the overlay window when reasserting its macOS topmost level, and tolerating one transient macOS tracker/helper miss before hiding the overlay.
+- Kept subtitle tokenization warmup one-shot for the lifetime of the app so later fullscreen/media churn on macOS does not replay the startup warmup gate after the first file is ready.
+- Added a bounded macOS tracker loss-grace window so fullscreen enter/leave transitions do not immediately hide and reload the overlay when the helper briefly loses the mpv window.
+- Skipped subtitle/tokenization refresh invalidation on character-dictionary auto-sync completion when the dictionary was already current, preventing startup flash/reload loops on unchanged media.

changes/2026-03-16-stats-storage-optimizations.md

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+type: changed
+area: immersion
+
+- Kept immersion tracking history by default while preserving daily/monthly rollup maintenance.
+- Added exact lifetime summary reads for overview/anime/media stats so dashboard totals no longer depend on rescanning raw telemetry.
+- Reduced tracker storage overhead by removing duplicated subtitle text from subtitle-line event payloads.
+- Deduplicated episode cover-art blobs through a shared blob store and updated cover-art reads/writes to resolve shared images correctly.
+- Added indexes for large-history session, telemetry, vocabulary, kanji, and cover-art queries to keep dashboard reads fast as the SQLite database grows.