`keepalive(path, ttl)` operation

[bigbes]

TbCS exposes keepalive both as a top-level API and as a transaction op. It extends the deadline of an existing expirable record:

• Errors if path is a prefix.
• Errors if ttl is +inf (cannot make a record permanent).
• Errors if the schema lacks the deadline index.
• Returns {ok = false, message = …} if the record does not exist or has no current deadline.
• Only grows the deadline — if the new deadline is ≤ the existing one, it is a no-op.
• Does not bump the global revision, by design — TTL-only updates must not fire watchers.

The wrapper has no operation type for it; the ops map is {get, put, delete} only.

etcd compatibility

LeaseKeepAlive(lease_id) — refreshes a lease, not a key. Mirroring etcd's lease API on top of TbCS would mean implementing the client-side lease emulation that the server-side etcd shim already does (in-memory {TTL, keys = {}} table; keepalive runs a txn that asserts every attached key still exists and refreshes each via keepalive). Recommend exposing native keepalive first; lease emulation can be a separate optional issue.