Keyspace indirection for timeline branches.

Every storage key embeds a database name. A branch created by checkpointing a parent database keeps the PARENT's name inside the copied keys, so the branch carries a keyspace (the name used for key building) distinct from its logical (registered) name. Normal databases have keyspace =:= name and no entry is installed.

resolve/1 is idempotent under the v1 invariant: a keyspace value is always the name of a database whose own keyspace is itself (branching a branch is rejected, lineage is linear). This lets internals pass an already-resolved name into helpers that resolve again.

Branch identity (keyspace, parent, fork_hlc) persists in a sidecar file TIMELINE inside the database directory, NOT in db_meta: the compaction filter starts before RocksDB opens, so identity must be readable without the store. The file is written atomically at fork (temp + rename) and never rewritten.