fix(tmux): pre-size destination window before switch-client

[vimeh]

Problem

With window-size latest, every tmux window keeps the size of the last client that viewed it. When the user's terminal size changes over the day (monitor moves, font zoom), switching into a session whose window was last viewed at a different size makes tmux rescale the whole window at attach time: panes are redistributed proportionally, so the sidebar balloons (observed 32 -> 48/56/103 columns) for a visible frame until the after-resize-pane width-repair hook snaps it back.

The user-visible effect is a double reflow on every switch into a stale-sized window: land on a garbled layout, main-pane TUIs start redrawing at the wrong width, then the sidebar snaps back. Measured live this was ~100-300ms of visual churn on ~20% of switches (95 width-repairs across 486 switches in one day's debug log), plus occasional repair storms when consecutive repairs re-trigger the resize hooks.

Fix

TmuxProvider::switch_session now pre-sizes the destination session's active window off-screen before switch-client:

1. Read the switching client's current window size and the destination's active window size (2 read-only tmux calls; this is the whole cost when sizes already match).
2. If they differ: resize-window the destination to the client's size, put the sidebar pane back to @opensessions_width, then switch.
3. After the switch, set-window-option -u window-size restores automatic size tracking, since resize-window pins the window to window-size manual. At that point the recomputed size equals the one just set, so this is never a visible resize.

Attach becomes a layout no-op; the existing width-repair hooks remain as a backstop.

Testing

• 3 new unit tests over a scripted command runner: command ordering (presize -> switch -> restore tracking), no-op when sizes match, no-op when the client is already on the target session.
• cargo test --workspace: unit suites green; tmux E2E 22/23 with the one failure (tmux_sidebar_stays_closed_across_session_and_window_switch_hooks) also failing identically on the unmodified base in this environment (host tmux config leaking into the test lab).
• Verified live: a 20ms-interval pane-width probe during sidebar-driven switches showed the 32->103->32 balloon before the fix and no non-32 samples after.

[inspect-review]

inspect review

Triage: 16 entities analyzed | 0 critical, 0 high, 9 medium, 7 low
Verdict: standard_review

Findings (0)

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Reviewed by inspect | Entity-level triage found 0 high-risk changes