With the v2→v3 migration sweeping stale `__run__*` branches off `__manifest`
on first read-write open, the defense-in-depth `is_internal_run_branch` guard
is no longer needed.
- delete `db/run_registry.rs`; drop the module + re-export from `db/mod.rs`
- collapse `is_internal_system_branch` to the schema-apply-lock check only
- `ensure_public_branch_ref`: drop the run-ref rejection; `__run__*` is now an
ordinary branch name
- `branch_merge`: reject `is_internal_system_branch` (was run-only) so the
schema-apply lock is rejected consistently with create/delete — a small,
deliberate tightening
- update the inline schema-apply test + the writes integration tests
(`public_branch_apis_reject_internal_run_refs` →
`public_branch_apis_reject_internal_system_refs`, which also asserts
`__run__*` now creates successfully)
- docs: flip the "pending production sweep / defense-in-depth" notes to
"auto-swept by the v2→v3 migration"; document the read-only-open limitation
Known residual: the inert `_graph_runs.lance` / `_graph_run_actors.lance` bytes
remain until a `StorageAdapter::delete_prefix` primitive lands.
