chore(node): finish Phase 3 — CLI deprecations, Windows shapes, docs

`member upload` reached the generic usage line for the other `member` verbs —
"usage: meshbay-node member upload <username>" — which advertises a removed
feature and sends the operator looking for a username it would then reject. It
names `root set --writable` now, and the man page carries the same. Three lines
between an operator finding the replacement and concluding the CLI is broken.

`--upload-dir` still works, so an existing script keeps working, but its help
and the man page say it is the old spelling and name what replaced it.

The Windows pass (§4.4) is what can be checked from here, made checkable:
drive letters and UNC through `as_posix()` into TOML, a drive root having no
basename to derive a name from — sharing a whole drive is ordinary there — and
a case-insensitive collision, which on NTFS and exFAT is one directory indexed
as two roots. `PureWindowsPath` throughout, for the reason the backslash test
earlier this branch got wrong.

What it cannot check is written down rather than glossed: ReadDirectoryChangesW
dropping events, MAX_PATH, and whether an eject actually lets a drive be
removed. §7d says so, along with two things the plan never considered — the
RO/RW asymmetry in `_do_dir_delete`, and `index_delta` carrying roots but not
`dirs`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pvMdvLBG92jyhvD5pD6us

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