(style): #main :has() rules make each terminal switch force ~2 s of style recalc #541

Two :has() rules added by #511 (#506) to public/style.css make every DOM insertion under #main invalidate style on the whole subtree:

• style.css:5543 #main:has(#terminal-area:not([style*="display: none"])) { min-width: var(--tool-main-floor) }
• style.css:5547 #main:has(#terminal-area:not([style*="display: none"]) #file-panel.open) { min-width: min(...) }

Measured live on main a02f96c (isolated test instance, 5 terminals with 30,000 lines each):

• Each tab switch was one ~2 s long animation frame, almost all of it style and layout (styleAndLayoutStart ≈ 1.9 s, no script attribution).
• CPU profile: 2163 ms of self time in xterm's CharSizeService _measure. Stack: showSession → suspendTerminalWebgl → WebGL addon dispose → DOM renderer renderRows → 59 × createRow → _measure.
• Every row forces a synchronous layout, and the :has() rules make each forced recalc cost 25–35 ms.

Before #511: 67–79 ms.

#532 hides most of it by keeping three warm WebGL contexts, but any path that still builds DOM rows under #main pays the cost: a cold terminal, a fourth terminal beyond the warm cap, the grid.

Fix: drive the two rules from a class toggled where #terminal-area display and #file-panel.open already change, e.g. #main.terminal-area-shown and #main.terminal-area-shown.file-panel-open, and drop :has() on #main. Keep the min-width behaviour #506 wanted.