refactor flamegraph TUI model into focused sub-controllers

Break the god-class Model in internal/tui/flamegraph/model.go into four
focused sub-controllers that each own a single concern:

- ZoomNavigator  : zoom path, stack, root node, and reset/undo logic
- SelectionManager: selected frame index, clamp, traversal, navigation
- FrameAnimator  : spring animation, frame layout swap, ancestry index
- SearchController: search query, match indices, filter-visible set

All four are embedded in Model so existing field access (m.selectedIdx,
m.zoomPath, etc.) is promoted unchanged, keeping tests and callers intact.
Model methods now delegate to the sub-controllers rather than holding all
logic inline.

Also split RenderTerminalView (88 lines) into computeRenderParams,
buildToolbar, buildFilteredStatus, and buildNormalStatus helpers, and
extracted buildSnapshotMsg from RefreshFromLiveTrieCmd.

All functions are now ≤ 50 lines. All tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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