fix(tabs): recompute grid rows when the tab strip toggles

[cmoron]

Summary

Opening a second tab (or closing back to a single tab) shows/hides the tab strip, which changes the terminal's top margin. The grid's row count was not recomputed, so the bottom rows overflowed off-window until the next window resize.

Reproduction

1. Launch Rio in the default Tab navigation (tab strip hidden while there is a single tab).
2. Fill the terminal to the bottom (e.g. seq 1 200, or open vim).
3. Open a second tab — the strip appears.
4. The content shifts down by the strip height, but the grid keeps the same number of rows, so the bottom row (shell prompt / bottom of vim) is now clipped below the window edge. The window itself did not resize.
5. Manually resizing the window corrects it.

Reproduced on main (2bb89d2); the report was cross-platform.

Root cause

ContextGrid represents the margin twice, and both must stay in sync:

• scaled_margin — the render offset (origin_y = panel_rect.y + scaled_margin.top).
• the Taffy root-node size — from which the panel height, and therefore the row count (lines, the PTY winsize), is derived. It is set by try_update_size, which subtracts scaled_margin.

ContextGrid::resize() (the window-resize path) updates both. But resize_top_or_bottom_line — the tab-strip toggle path — called update_scaled_margin() (which sets only the field) followed by resize_all_contexts() (which resends the existing, now-stale dimension). The Taffy root was never resized, so lines stayed at the full-height value: the content shifted down without dropping any rows, and only the next window resize (the one path that runs try_update_size) corrected it.

Fix

After updating the margin, re-run the same per-grid resize the window-resize path already uses (try_update_size + apply_taffy_layout), so the root shrinks by the new margin and the row count is recomputed and pushed to the PTY. This covers both directions: the strip appearing (1 → 2 tabs) and disappearing (2 → 1).

Testing

• cargo test -p rioterm — 147/147 pass.
• Verified manually on Linux (WSLg) with before/after builds: opening a tab no longer clips the bottom, and closing back to a single tab refills the grid instead of leaving a gap at the bottom.

[cmoron]

Closing as a duplicate. This is #1495, already addressed by #1632 (fixes at update_scaled_margin and adds a regression test — closes the issue) and #1687. I reached the same root cause independently — the terminal margin is tracked both in the scaled_margin field and in the Taffy root-node size, and only resize()/try_update_size kept them in sync — but those PRs got there first, so no need for a fourth. Thanks!