feat(tui): animate focused pane borders

[edwin-zvs]

Summary

• animate newly focused session-list, lineage, and split-pane borders with a 200 ms top-left to bottom-right sweep
• keep borderless zoom and hidden-edge layouts unchanged while using a dedicated animation-only frame cadence
• document the focus affordance and cover timing, sibling-window retriggering, diagonal phase, and top-only chrome in tests

Verification

• cargo test -p construct-cli focus_border (4 passed)
• cargo build (passed; construct is the workspace binary)
• cargo test -p construct-cli (1419 passed, 1 pre-existing environment failure: color::tests::truecolor_backend_still_emits_rgb; the same test fails unchanged on origin/main because this runner suppresses ANSI color output)
• VHS after-only recording captured and frame-verified; attachment follows in a PR comment

[edwin-zvs]

After-only VHS demo (full borders enabled): focus cycles across two split windows and the session list. The 200 ms high-contrast tracer moves from top-left toward bottom-right on each newly focused surface.

[edwin-zvs]

Updated after-only VHS demo: steady side/bottom pane borders are hidden. Focus briefly draws the full perimeter as a foreground-only diagonal line sweep, including all four corners; no reverse/background highlight.\n\n