Ports the fix from PR #255 to current master (which had refactored
render_json -> render_layout_json to module level).

For 3+ pane layouts, the previous "both_leaves" half-highlight path
incorrectly colored separators between non-active leaf children as if
their adjacent leaf were active, regardless of whether the active pane
was actually elsewhere in the tree.

This change:
* Adds LayoutJson::count_leaves() to compute total pane count.
* Threads total_panes through render_layout_json and gates the legacy
  half-highlight path with `total_panes == 2` so it only applies to
  simple two-pane splits.
* For 3+ panes, separator cells are colored per-cell based on adjacency
  to active_rect (the existing else branch), giving correct highlighting
  for arbitrary nested layouts.
* When zoomed, total_panes is forced to 1 to skip traversal.
* Restricts the post-render re-coloring pass to junction characters
  (┼ ├ ┤ ┬ ┴) only. Straight │ and ─ separators are now already colored
  correctly per-cell by render_layout_json, so re-coloring them was
  clobbering the new behavior for 3+ pane layouts.

Also updates the two other call sites (preview.rs choose-tree previews
and main.rs render-test diagnostic) to pass total_panes.

Tests:
* tests-rs/test_pr255_active_border.rs - 7 unit tests covering
  count_leaves for 1/2/3/4/5-pane layouts and a TestBackend render
  regression test asserting that for a 3-pane H[active, V[inactive,
  inactive]] layout, no '─' cell on the right side is colored as active.
* tests/test_pr255_active_border.ps1 - 12 PowerShell E2E + TUI
  verification tests covering 1/2/3/4-pane creation, select-pane on
  each pane, geometry, zoom toggle, and capture-pane on all panes.

Co-authored-by: LizardLiang <shotup0101@gmail.com>

Thanks @LizardLiang for the contribution!

Ports the fix from PR #255 to current master (which had refactored
render_json -> render_layout_json to module level).

For 3+ pane layouts, the previous "both_leaves" half-highlight path
incorrectly colored separators between non-active leaf children as if
their adjacent leaf were active, regardless of whether the active pane
was actually elsewhere in the tree.

This change:
* Adds LayoutJson::count_leaves() to compute total pane count.
* Threads total_panes through render_layout_json and gates the legacy
  half-highlight path with `total_panes == 2` so it only applies to
  simple two-pane splits.
* For 3+ panes, separator cells are colored per-cell based on adjacency
  to active_rect (the existing else branch), giving correct highlighting
  for arbitrary nested layouts.
* When zoomed, total_panes is forced to 1 to skip traversal.
* Restricts the post-render re-coloring pass to junction characters
  (┼ ├ ┤ ┬ ┴) only. Straight │ and ─ separators are now already colored
  correctly per-cell by render_layout_json, so re-coloring them was
  clobbering the new behavior for 3+ pane layouts.

Also updates the two other call sites (preview.rs choose-tree previews
and main.rs render-test diagnostic) to pass total_panes.

Tests:
* tests-rs/test_pr255_active_border.rs - 7 unit tests covering
  count_leaves for 1/2/3/4/5-pane layouts and a TestBackend render
  regression test asserting that for a 3-pane H[active, V[inactive,
  inactive]] layout, no '─' cell on the right side is colored as active.
* tests/test_pr255_active_border.ps1 - 12 PowerShell E2E + TUI
  verification tests covering 1/2/3/4-pane creation, select-pane on
  each pane, geometry, zoom toggle, and capture-pane on all panes.

Co-authored-by: LizardLiang <shotup0101@gmail.com>

Thanks @LizardLiang for the contribution!

…view

Renders the same render_layout_json the live TUI uses to a TestBackend
(via the hidden _render-preview command), parses the ANSI output, and
proves per-cell color correctness for a 3-pane H[active=%1, V[%2, %3]]
layout when each pane is activated in turn.

8 assertions including:
* Inner horizontal separator (between inactive %2 and %3) has 0 active
  cells when %1 is active. Before PR #255, half would be green.
* When %2 is activated, the same separator becomes fully active.
* Outer vertical separator: per-cell adjacency works (upper half active
  when %2 is active, lower half inactive).