New docs/core/filesystems.mdx covering per-filesystem capabilities
(CoW, pool-awareness, visibility filtering, capacity correction),
platform detection strategies, the FilesystemHandler trait, Linux/
macOS/ZFS visibility rules, the ZFS pool-root capacity problem and
fix, copy strategy selection, and known limitations.
Registered under File Management in both mint.json and docs.json.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
