Adds the foundation for working OSC 8 hyperlinks in the transcript:
- LinkRegion struct: (row, col_start, col_end, target) for a contiguous
run of linked cells on one terminal row
- write_osc8_open/close: emit OSC 8 escapes directly through a Write
impl (bypassing ratatui's buffer which strips ESC bytes)
- FRAME_LINKS thread-local: passes link regions from the render closure
to ColorCompatBackend::draw(), where OSC 8 escapes are emitted
out-of-band through the backend's Write impl
- ColorCompatBackend integration: draw() reads FRAME_LINKS, emits OSC 8
open/close around linked cells
The markdown renderer still uses the inline Span::content approach
(known broken); the sentinel-color buffer-scan integration is a
follow-up. This PR delivers the emission path and thread-local
plumbing so the remaining work is confined to link detection in the
render closure.
Hunter Bown
60e9f706b3