fix(native): ignore terminal calls after Close

[omartelo]

Follow-up to #780, from the concern raised in #777: a light/dark switch during a reconnect could paint terminals that had just been freed.

Cause

terminals.reattach runs on the reconnect goroutine and calls Close on every terminal. Since #780, setColors snapshots the views under ts.mu and paints them after releasing it, so a SetColors can land on a terminal reattach freed a moment before. A frame holding the shown terminal across the swap (show then layout) had the same exposure, as did onOutput's Write. gioterm.Terminal documents its methods as safe from several goroutines, but Close left v.t dangling and the next call ran on freed libghostty memory.

Fix

• Close marks the Terminal closed. Every method after it does nothing: it returns ErrClosed where it returns an error and its zero value otherwise. A second Close is a no-op.
• A layout error ends the event loop, so window.content treats ErrClosed as a frame to redraw: the next frame's show picks up the reattached terminal.

Tests

• TestClosedTerminalIgnoresCalls calls every method on a closed terminal. Before the fix, SetColors ran on the freed terminal and returned nil. After it, the test passes with -race -count=50.
• native/: gofmt -l ., go vet ./..., go test ./... clean.
• No test drives the ErrClosed branch in window.content. Not checked in a real window.

Refs #777