Frames the fix for unusable large-file editing in pmacs-gpu: render
only the visible byte slice (O(visible) not O(file)) + line-based
scroll. Stance: feed cosmic-text only current_text[vstart..vend] (the
native Scroll path only makes shaping lazy, not set_rich_text /
projected_rich_chunks, which dominate). Q-decisions: line-based scroll,
caret-follow auto-scroll, small overscan, rebase-by-vstart, scoped
Viewport declaration; daemon whole-file highlight query deferred
(Q#S6). Bet S2 (coordinate-space rebasing) flagged as the QB3-class
risk. Fact-checked: all load-bearing claims hold (GPU declares
whole-file viewport; reshape is O(file); producer already clips spans
to vp.visible; cosmic-text splits BufferLines on \n so slices must be
line-aligned; caret/wash builders use whole-file offsets).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
