fix(tui): keep focus sweep visible on borders

[edwin-zvs]

Summary

• compose the moving focus sweep as a border-glyph toggle, erasing cells where a pane or Playbook border is already visible
• retain the bright drawn segment on hidden side and bottom edges without changing animation timing, geometry, or border visibility semantics
• add phase-frozen frame regressions for top-only chrome, full Playbook or /border frames, hidden edges, corners, and title preservation
• update the focused UX spec to record the visible-versus-hidden composition rule

Follow-up to #1278.

Verification

• cargo test -p construct-cli focus_border -- --nocapture (7 passed)
• cargo build (passed)
• git diff --check (passed)
• cargo fmt --all -- --check reports a pre-existing repository-wide rustfmt delta across untouched crates; no formatting changes were applied outside this focused diff

Visual verification

The TestBackend regressions freeze the 200 ms animation at an exact phase and render real Ratatui Block frames before applying the sweep. They verify that the same moving cells are blank over an always-visible top rule or full Playbook or /border frame, and bright line glyphs over hidden edges. A trustworthy before/after VHS pair would require the second pre-change worktree mandated by AGENTS.md, while this task explicitly disallows creating another worktree, so the exact frame assertions are the more deterministic artifact here.