speed up flame graph TUI under heavy event load

Move the per-tick snapshot refresh off the Bubble Tea update goroutine,
add a frame ancestry index so navigation and filter helpers run in
O(subtree) instead of O(frames), skip refresh and animation while the
user is actively pressing keys, and memoize View() output. Keystrokes
(pause, filter, navigate) now land within one frame even when the live
trie ingests thousands of events per tick.

- new SnapshotTree() on LiveTrie bypasses JSON marshal+unmarshal
- RefreshFromLiveTrieCmd runs SnapshotTree + layout + ancestry on a
  background goroutine, coalesced via refreshInFlight, and returns a
  flameSnapshotReadyMsg the Update loop applies cheaply
- driveWindow gate (250 ms after last key press) skips refresh dispatch
  and snaps frames directly to target without animation while the user
  is driving
- View() caches its rendered string keyed on the inputs that affect
  output; cache is bypassed during animation

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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