feat(store): the data root under LOCALAPPDATA with an owner-only DACL, one lock seam, MoveFileEx (P11c)

Windows step 1 and 2 of D-62. Where Unix keeps the books' private folder
at mode 0700 and refuses one others can open, Windows now makes the same
promise with a DACL: `winsec::set_owner_only` gives the folder (and,
inherited, everything made in it) one protected allow entry for this
account's SID; `winsec::is_private` reads the effective DACL and owner
back and answers false for anything else (a missing DACL, an entry for
Everyone), so `Root::open` refuses a loosened root and `Root::setup`
makes it private again, exactly as the 0700 checks do. Every question the
system cannot answer is "not private". The root itself comes from the
shell's known folders (`SHGetKnownFolderPath`: the profile, and
LocalAppData wherever it was redirected), never from environment
variables the process that started us chose.

`rename_new` (the move that must never go over what is there, renameat2
RENAME_NOREPLACE on Linux) is `MoveFileExW` without the replace flag on
Windows, written through.

File locking sat behind `libc::flock` in ops.rs alone (the rest already
used std's File::try_lock, which is LockFileEx on Windows). It now goes
through `lock.rs`, which uses only std, so there is no second unsafe copy
of either call.

New dependency: windows-sys 0.61 behind cfg(windows), the raw bindings
only (no runtime), already in the lockfile through other crates; the
smallest Microsoft crate that covers SIDs, DACLs, the known-folder API,
pipe process ids and MoveFileExW. mise.toml adds the Windows target to
the pinned toolchain so `cargo check --target x86_64-pc-windows-msvc`
type-checks this arm on Linux.

CHOICE (P11c): nothing above the root is checked on Windows (the Unix
rule about folders above it being swappable has no Windows authority; the
profile's folders belong to the account and the Administrators).

Tests: tests/windows_root.rs (Windows job only), the cross-platform
`a_viewer_s_presence_is_a_shared_lock_others_can_see` in ops.rs.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

Cargo.lock

@@ -4049,6 +4049,7 @@ dependencies = [
  "subtle",
  "tempfile",
  "thiserror 2.0.21",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]