Add opt-in xterm.js 6 capture modes

[alsi-lawr]

Proposal: keep the existing xterm.js 5 layered capture as the default (legacy), and add two opt-in xterm.js 6 modes:

• record: record the composed terminal canvas, then feed decoded frames through the existing render pipeline.
• webgl: capture the composed WebGL terminal frame directly.

This avoids DEC synchronized-output (CSI ? 2026 h/l) blank-frame flashes without removing the current compatibility path. vhs serve gets the same flag.

Working implementation and docs: alsi-lawr/vhs@feat/xterm6-capture-modes

In the spike, WebGL had 0/100 captures over the 20 ms frame budget (p95 5.77 ms), and both new modes produced zero uniform blank frames in the DEC and Codex tapes.

Would this shape be acceptable for a PR?

[alsi-lawr]

Generated outputs are available on an examples-only branch for inspection: xterm6-capture-examples. It contains the minimal DEC and Codex GIF/WebP captures for legacy, record, and webgl.