#6042 Fix stale rendering after shared texture atlas page merges

Part of: https://github.com/xtermjs/xterm.js/issues/6038
Related: https://github.com/microsoft/vscode/issues/322756

• Add a focused Chromium WebGL regression test for terminals sharing one texture atlas.
• Keep each terminal rendering correctly when another terminal sharing the same texture atlas triggers a page merge.
• Replace the shared consume-once atlas invalidation signal with a monotonic pageLayoutVersion.
• Track _lastSeenPageLayoutVersion per GlyphRenderer so every WebGL renderer independently refreshes cached glyph page mappings.
• Reset the renderer's last-seen page layout version when its atlas is replaced.

TDD:

• 780cbef3 fails the focused repro with rendered signature drift while terminal B's buffer text remains correct.
• 9559005 passes the same focused Chromium WebGL integration test.

Verification:

• npm run build
• npm run esbuild
• npm run test-integration -- --suite=addon-webgl --project=Chromium addons/addon-webgl/test/WebglSharedAtlasGarble.test.ts
• npm run lint-changes

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Inspirations from:

• The full-model refresh behavior after atlas changes comes from WebglRenderer.renderRows.
• The stale shared-state bug is in the old consume-once atlas invalidation contract: TextureAtlas.beginFrame, page merge invalidation, and oversized-page invalidation.
• The renderer-side check preserves the existing GlyphRenderer.beginFrame call site while moving acknowledgement state per renderer: GlyphRenderer.beginFrame.