Introduce the ability to reveal specific sections of a document
rather than entire files. This includes:
- Updating `revealedPaths` to map normalized file paths to sets of
revealed section slugs.
- Adding `isPathRevealed` to handle visibility logic for connected
clients and GMs.
- Enhancing `extractHeadings` to calculate `startLine` and `endLine`
for each TOC node to support precise section identification.
hypercross
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