Group comment threads on one block under a single gutter marker

Each block with open threads now gets one marker in the right gutter,
centred on its first line. Several threads show a count and open a
compact list; selecting one opens the thread with a way back. On narrow
documents the chip narrows to fit the content padding instead of
covering text. The open card renders once after every marker, and other
markers dim and become inert while it is open.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>