PR #6043 - xtermjs/xterm.js

Keep the existing exact-four, same-sized atlas page merge behavior.
Evict and rebuild the glyph atlas when it reaches maxAtlasPages and that merge is unavailable, instead of adding a page the renderer cannot use.
Apply the same capacity check before creating an oversized-glyph page.
Increment pageLayoutVersion after eviction so every renderer sharing the atlas rebuilds its cached glyph mappings.
Clamp the texture upload loop as a defensive backstop so an unexpected overflow cannot throw while rendering.
Add deterministic Chromium coverage for normal page overflow, oversized glyphs, shared terminals, rendered pixels after eviction, and mid-frame retry stability.

TDD evidence:

Red: 8237004d5 reproduces both overflow paths on the PR 1 baseline with TypeError: Cannot read properties of undefined (reading 'version').
Green: 2b982bc39 keeps the atlas within renderer capacity and passes both tests.

Steps to verify:

Inspirations from:

Preserving exact-four merging comes from the guard added to fix the earlier stack overflow in TextureAtlas._createNewPage.
The capacity invariant comes from GlyphRenderer allocating its shader and texture arrays from TextureAtlas.maxAtlasPages, including one texture per supported page in the _atlasTextures initialization.
The eviction cache reset follows the existing cache-clearing behavior in TextureAtlas.clearTexture, while additionally removing the discarded pages.
Handling oversized glyphs at the same capacity boundary comes from the existing direct _overflowSizePage insertion path in TextureAtlas.
Mid-frame eviction recovery relies on the existing full-model retry behavior in WebglRenderer.renderRows.

#6043 Prevent WebGL atlas pages from exceeding texture capacity