(terminal): stale WebGL glyph atlas garbles rows whose buffer text is correct #502

What happens

A Claude session's terminal shows garbage glyphs over whole rows (the first-run welcome art, the menu below it) while plain-ASCII rows on the same screen render correctly. Seen on the v0.0.90 candidate (PR #500, Windows 11, Intel Iris Plus via ANGLE D3D11, DPR 2, 1400x900).

Measured on the live instance (read-only, through the debug port)

• The xterm buffer holds the correct text ("Welcome to Claude Code v2.1.295", the block-art characters, the theme menu): the bytes are fine, the corruption is in the WebGL renderer.
• WebGL addon active (@xterm/addon-webgl 0.19.0, @xterm/xterm 6.0.0, Electron 41.0.3), MAX_TEXTURE_SIZE 16384, cell 7x14 css / 14x28 device (integer).
• webglAddon.clearTextureAtlas() + terminal.refresh(0, rows-1), run once, repaired every glyph.
• The existing heal paths did not fire a repair: loadTerminalWebgl repaints on onChangeTextureAtlas / onAddTextureAtlasCanvas, and forceRepaint clears the atlas on reveal. So the atlas went bad without a change event, or after the last repaint.
• No @xterm/* or Electron bump in six weeks; the only dependency change in the path is node-pty 1.2.0-beta.15 ((terminal): move node-pty to 1.2.0-beta.15 (#409) #431), unrelated to a renderer-side atlas.

Hypotheses (to decide by measurement)

1. Atlas corruption in addon-webgl 0.19 at DPR 2 on ANGLE D3D11 when many new box/block glyphs arrive at once. Decide: with the activity trace on, count atlas events on a fresh first-run screen; same buffer with the WebGL addon disposed (DOM renderer).
2. Two WebGL contexts created at nearly the same time (main terminal + panel shell), as in (terminal): a resize while another terminal's WebGL context starts can paint it blank #353. Decide: create them sequentially and compare.
3. Texture upload on first paint while fonts settle. Decide: read the atlas canvas before clearing it.

Wanted

The measurement first, then a fix that heals without user action (e.g. clear the atlas when a burst of new glyphs lands, or on a detected mismatch), with a test that loads the shipped terminal-manager.js. The Refresh control already clears the atlas: say whether it does, as a manual workaround meanwhile.