9p file sharing

[redbeam]

Note

Together with this PR, we also need to merge
crc-org/snc#1123 to add the 9pfs client to the bundle.

• 9p: add 9p fuse client snc#1123

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This adds 9p file sharing support as discussed in #4858.

Summary by CodeRabbit

• New Features
  • Windows can expose the user home via a built-in 9P file server; guests can mount host dirs with a new "9p" shared-dir option.
• Bug Fixes
  • More robust network listener/address handling and improved mount error reporting; sensitive data masked in errors.
• Chores
  • Vendored a cross-platform 9P implementation (client/server/protocol) and integrated 9P server lifecycle into startup.
• Tests
  • Added end-to-end tests verifying 9P mounts and basic file/directory operations on Windows.
• Packaging
  • Installer now registers HVSOCK support for 9P on Windows.

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