[codex] add WebDAV-backed TP-7 mount

[totocaster]

Summary

Adds a first Finder-visible tp7 mount implementation without requiring macFUSE, Fuse-T, kernel extensions, system extensions, or FieldKit.

The mount command now starts a loopback-only WebDAV bridge backed by a long-lived TP-7 MTP session, then asks macOS mount_webdav to expose it as a Finder volume. With no mountpoint argument, it chooses /Volumes/TP-7, then /Volumes/TP-7 2, etc. A new tp7 unmount command tears down one mount when a mountpoint is provided, or all TP-7 mounts recorded by the CLI when no argument is passed.

What Changed

• Added tp7 mount [mountpoint] and tp7 unmount [mountpoint] command wiring.
• Added a loopback WebDAV server that supports read-only Finder access through OPTIONS, PROPFIND, HEAD, and GET.
• Backed file reads with MTP partial reads when available, with full-object stream fallback when the device rejects partial reads.
• Added mount state files under the user application support directory so tp7 unmount can discover active CLI-owned mounts.
• Added mount/unmount human and JSON output.
• Updated README and protocol docs to describe the WebDAV mount model and the current read-only boundary.

Why

The goal is to expose the TP-7 filesystem as a folder/drive on macOS without shipping a filesystem extension. FieldKit can present a Finder-visible drive, but the CLI should stay independent of companion apps and avoid macFUSE/system-extension requirements. macOS' built-in WebDAV client gives us a native mount path that can be driven from a standalone Rust CLI.

Read-Write Boundary

This PR intentionally keeps the Finder mount read-only. MTP is object-based rather than block-based, so Finder/POSIX write behavior needs more than accepting PUT: safe support requires staged whole-file writes, upload-on-close semantics, safe-save rename handling, delete/move mapping, metadata policy for .DS_Store and AppleDouble files, and a decision for TP-7 folder creation behavior because firmware 1.1.9 rejected MTP folder creation in earlier hardware smokes.

The --read-write flag exists only to fail clearly for now.

Validation

Ran:

• cargo fmt -- --check
• cargo check
• cargo clippy -- -D warnings
• cargo test
• cargo run -- --help
• cargo run -- mount --help
• cargo run -- mount --read-write
• cargo run -- devices

Hardware mount validation was not run because cargo run -- devices reported no TP-7 devices found on this machine at PR creation time.