fix(webgl): release the GL context on dispose

[openwong2kim]

Fixes #6068

What

Explicitly lose the WebGL2 context when the WebGL renderer is disposed, by
calling WEBGL_lose_context.loseContext() in the renderer's teardown:

this._canvas.parentElement?.removeChild(this._canvas);
removeTerminalFromCache(this._terminal);
// Detaching the canvas does not free the underlying WebGL2 context; it
// lingers until it is garbage collected. Browsers cap the number of live
// WebGL contexts per document (Chromium ~16), so in apps that create and
// dispose many terminals these orphaned contexts accumulate and the
// browser force-evicts the oldest LIVE context, blanking an active
// terminal. Explicitly lose the context so it is released immediately.
this._gl.getExtension('WEBGL_lose_context')?.loseContext();

Why

WebglAddon.dispose() detached the canvas but never released the underlying
WebGL2 context. It lingered until garbage collection, so every disposed addon
left a zombie context behind. Browsers cap the number of live WebGL contexts
per document (Chromium ~16, driver/build dependent); once an app creates and
disposes enough terminals to cross the cap, the browser force-evicts the
oldest context — which can belong to a terminal that is still on screen — and
that live terminal goes blank. See #6068 for a repro and full
analysis.

loseContext() is the standard, documented way to release a context
deterministically. getExtension('WEBGL_lose_context') is optional-chained so
a browser that does not expose the extension simply degrades to the previous
(leak-on-GC) behavior instead of throwing.

Tests

Added addons/addon-webgl/test/WebglContextRelease.test.ts, an integration test
that spies on WEBGL_lose_context.loseContext and asserts dispose() invokes
it exactly once. The cap-eviction itself is GPU/driver dependent and does not
reproduce under headless SwiftShader, so the test verifies the mechanism
directly rather than trying to force an eviction. Verified it fails without this
change (loseContext called 0 times) and passes with it.

• npm run tsc — clean
• eslint on the changed files — clean
• New test passes on Chromium; the pre-existing shared-renderer color-comparison
  failures under headless SwiftShader are unaffected by this change (they fail
  identically on master).