Replace flaky per-window screencapture loop in Notion manual demo recorder

[schickling-assistant]

Problem

The manual demo recorder for @overeng/notion-react now has working chapter transitions, validation, sync summaries, and overlay composition, but the current capture backend still relies on a per-window screencapture polling loop.

In practice this is flaky on macOS:

• screencapture can intermittently print could not create image from window
• successful end-to-end runs can collapse to slideshow-level input rates (for example ~29 captured frames over ~160 seconds)
• the resulting video can validate logically while still being visually unusable

Desired outcome

Replace the current per-window PNG polling loop with a stable capture strategy that:

• records only the dedicated Ghostty + Chrome demo area
• does not hijack user focus or dim the rest of the screen
• produces consistent real-time frame rates suitable for a polished demo video
• keeps the existing overlay / manifest / validation pipeline intact

Likely directions

• record a fixed rectangle covering the two demo windows instead of polling each window separately
• or use a more stable native / ffmpeg-based capture backend than repeated screencapture -l...
• keep the output compatible with the existing chapter overlay compositor

Context

Current work already landed the following pieces in the recorder pipeline:

• empty-page preroll / pre-stage
• live tmux-driven editor transitions
• sync summary lines with timing and notion_api_calls
• bottom-band chapter overlays with progress indicator
• chapter-by-chapter Notion/browser/terminal validation

The next step is specifically capture reliability / frame rate.