perf(search): adaptive debounce and background matching for in-note search

[MiMoHo]

Problem

Fixes #1729. In long notes, in-note search starts searching after the very first
typed letter. Counting, nth-occurrence lookup and highlighting all ran synchronously
on the main thread, so a single-letter query with thousands of matches freezes the
UI for seconds (see the "&"-prefix workaround described in #1729).

Changes

1. Adaptive debounce instead of the static 50/200 ms delays: the delay scales
   with the measured duration of the previous search + highlight pass —
   clamp(2 × lastDuration, 50 ms, 750 ms), plus 150 ms for queries ≤ 3 characters.
   Short notes keep the current snappy behavior (addressing the earlier concern in
   Slow search in long notes #1729 that a fixed 1 s delay would hurt every use case), while long notes
   automatically get a delay long enough to let the user finish typing. Measuring
   real duration is more robust than scaling by content length, because the cost is
   dominated by the number of matches.
2. Background matching with cancellation: occurrence counting runs on a
   background executor; a generation counter discards superseded results ("some kind
   of Cancel event", as suggested by the maintainer in Slow search in long notes #1729). Only the newest query
   is highlighted, on the main thread.
3. No more full copies per query: jumpToOccurrence() allocated a full
   toLowerCase() copy plus a substring(0, index) copy of the entire note per
   query. Replaced by a single case-insensitive Matcher pass; the recursive,
   case-sensitive indexOfNth is gone (it also disagreed with the case-insensitive
   counting/highlighting).

Not covered

The remaining per-keystroke cost of the editor itself lives in
nextcloud-commons:markdown — see the companion PR
stefan-niedermann/nextcloud-commons#436 (executor reuse in
EditorStateNotifier, debounced SearchHighlightTextWatcher). Bumping
commonsVersion after its release will address #2489 / #3162.

Testing

• Existing unit tests pass (./gradlew test).
• Manual: note with ~100 kB text; type a common letter ("e") in the search bar and
  continue typing — the UI no longer freezes after the first letter; searching in a
  short note behaves as before.

[codacy-production]

Up to standards ✅

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