Adiantum encrypting VFS. (#77)

vfs/adiantum/README.md

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+# Go `"adiantum"` SQLite VFS
+
+This package wraps an SQLite VFS to offer encryption at rest.
+
+> [!WARNING]
+> This work was not certified by a cryptographer.
+> If you need vetted encryption, you should purchase the
+> [SQLite Encryption Extension](https://sqlite.org/see),
+> and either wrap it, or seek assistance wrapping it.
+
+The `"adiantum"` VFS wraps the default SQLite VFS using the
+[Adiantum](https://github.com/lukechampine/adiantum)
+tweakable and length-preserving encryption.
+
+In general, any HBSH construction can be used to wrap any VFS.
+
+The default Adiantum construction uses XChaCha12 for its stream cipher,
+AES for its block cipher, and NH and Poly1305 for hashing.
+It uses Argon2id to derive keys from plain text.
+
+> [!IMPORTANT]
+> Adiantum is typically used for disk encryption.
+> The standard threat model for disk encryption considers an adversary
+> that can read multiple snapshots of a disk.
+> The security property that disk encryption provides is that
+> the only information such an adversary can determine is
+> whether the data in a sector has or has not changed over time.
+
+The VFS encrypts database files, rollback and statement journals, and WAL files.