Artifact db_read/db_write gain an as_level run-as parameter with server-confirmed enforcement

What

• db_read and db_write calls can now include an asLevel parameter (returned as as_level), letting a call ask to run as if at a different access level.
• Results now include as_level_confirmed (from the server's levelConfirmed field), reporting whether the server actually enforced the requested level; when unconfirmed, a caveat message is added.
• The batch-write fallback path also tracks this confirmation across a sequence of writes, and appends a warning to error messages when a requested level wasn't confirmed, noting the writes may have run at the caller's own level instead.
• The db_write validation schema was updated to accept the new as_level (max 16 characters) and as_level_confirmed fields.

Why This makes it clear when an artifact database operation requesting an elevated or different access level actually got that treatment from the server, instead of silently succeeding or failing under the wrong permissions.