security: create key files with 0o600 permissions atomically

[OscarOzaine]

The problem

CBORSerializable.save() (used to write .skey key files) had three weaknesses:

1. World-readable files — files were created with the process umask (often 0o644), so other local users could read your secret keys.
2. TOCTOU race — it checked "does the file exist?" and then opened it as a separate step, leaving a window an attacker could exploit.
3. Symlink following — it would happily write through a pre-existing symlink.

The fix

save() now creates the file atomically with os.open(path, O_WRONLY | O_CREAT | O_EXCL, mode):

• O_EXCL makes creation atomic — it fails if the path already exists (closing the race) and refuses to follow a pre-existing symlink.
• The file is then chmod-ed to the requested mode regardless of umask.
• Default mode is 0o600 (owner read/write only).
• On any write failure, the partially-created file is cleaned up.

A new mode keyword lets callers of non-secret objects opt into wider permissions when they need it.

Behavior changes to be aware of

• save() now refuses to overwrite any existing file (previously it only rejected non-empty files — an empty pre-existing file used to be allowed).
• POSIX file modes are advisory on Windows; owner-only enforcement is POSIX-only. Windows users should rely on NTFS ACLs or disk encryption. This is now documented in the address guide.

Tests

All 574 tests pass. Tests were migrated from NamedTemporaryFile to pytest's tmp_path (so they don't pre-create the target file, which the new O_EXCL behavior rejects). New tests cover the 0o600 mode, overwrite refusal, and partial-write cleanup.