fix: remove document listeners on dispose

[camflan]

Problem

dispose() leaves two listeners on document:

• SelectionManager adds an anonymous mousedown listener (it tracks where a click started). Nothing keeps a reference to it, so dispose() cannot remove it.
• Terminal.dispose() sets isOpen = false before cleanupComponents(), and cleanupComponents() removes the mouseup listener only if (this.isOpen …). So that listener stays too.

The mousedown listener keeps the selection manager. The selection manager keeps the renderer, and the renderer keeps the canvas:

document ──"mousedown" listener
   └─► SelectionManager ─► CanvasRenderer ─► <canvas> (+ its GPU backing store)

An app that opens a new terminal each time the user moves to another one keeps every canvas. We found this in an Electron app that disposes a pane and opens a new one on each session switch. After four days the window held about 480 canvases, 12 GB of IOSurface in the GPU process. Blink counts canvas memory toward V8's global allocation limit, so V8 ran full GCs without a stop and the renderer used about 700% CPU.

Fix

• Store the mousedown handler as boundDocumentMouseDownHandler, and remove it in dispose() like the other document handlers.
• Remove the mouseup listener without the isOpen check. Removing a listener that was never added does nothing.

Test

lib/terminal.test.ts gets a test that records each listener added to document while a terminal opens, and checks that dispose() removes all of them. Before the fix, 2 are left.

bun run fmt, bun run lint and bun run typecheck pass. I ran bun test against the 0.4.0 ghostty-vt.wasm, because I did not build the WASM locally. With that file, one test fails, viewport scroll-growth corruption, and it fails with and without this change: it needs the WASM patch from #180.

[CaptainMittens]

A second symptom of the same leak, from the Nimbalyst desktop app (Electron 43, ghostty-web@0.4.0-next.20), in case it helps this get merged.

Nimbalyst gives each terminal its own Ghostty instance (because of #141), so each terminal also has its own WebAssembly.Instance. The mousedown listener keeps the SelectionManager, and SelectionManager.wasmTerm keeps that instance and its memory. One Electron renderer can hold about 122 live ghostty-vt instances. After that, WebAssembly.instantiate throws:

RangeError: WebAssembly.instantiate(): Out of memory: Cannot allocate Wasm memory for new instance

So in an app that rebuilds a terminal on each project switch, every new terminal in that window fails to start after roughly 60 switches, until a restart. See nimbalyst/nimbalyst#1818.

Check in a real Electron BrowserWindow: open a Terminal, write to it, dispose() it, and repeat. Without this change the loop failed after 123 terminals. With the same two listener changes as this PR, 400 terminals opened and closed with no error.

We ship the same change as a pnpm patch for now (nimbalyst/nimbalyst#1819). The patch also sets terminal, renderer and wasmTerm to undefined in SelectionManager.dispose(), so a reference held anywhere else does not keep the engine alive either. We will drop the patch once a release includes this fix.