refactor(engine): remove the legacy __run__ branch guard (MR-770)

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.

docs/dev/writes.md

@@ -14,8 +14,11 @@ publisher's row-level CAS on `__manifest` is the single fence.
 
 - No `RunRecord`, no `_graph_runs.lance`, no `_graph_run_actors.lance`.
 - No `omnigraph run *` CLI subcommands and no `/runs/*` HTTP endpoints.
-- No `__run__<id>` staging branches. (Legacy on-disk artifacts from
-  pre-MR-771 repos are inert; MR-770 sweeps them in production.)
+- No `__run__<id>` staging branches; `__run__*` is no longer a reserved
+  name. The branch-name guard was removed in MR-770, and any stale
+  `__run__*` branch on an upgraded graph is swept off `__manifest` by the
+  v2→v3 internal-schema migration on first read-write open. (The inert
+  `_graph_runs.lance` bytes remain until a `delete_prefix` primitive lands.)
 - Cancelled mutation futures leave **no graph-level state** — only orphaned
   Lance fragments, which the existing `omnigraph cleanup` pipe reclaims.
 

docs/user/audit.md

@@ -4,4 +4,4 @@
 - `_as` variants of every write API let callers override the actor: `mutate_as`, `ingest_as`, `branch_merge_as`, `apply_schema_as`, etc.
 - Actor IDs are persisted on `GraphCommit.actor_id` with split storage in `_graph_commit_actors.lance` (the commit graph is split into `_graph_commits.lance` for the linkage and `_graph_commit_actors.lance` for the actor map).
 - HTTP server uses the bearer-token actor automatically; CLI uses the local user / explicit env (no implicit actor).
-- Pre-v0.4.0 graphs also stored actor IDs on `RunRecord.actor_id` in `_graph_runs.lance` / `_graph_run_actors.lance`. The Run state machine was removed in MR-771; those files are inert post-v0.4.0 and reclaimed by MR-770's production sweep.
+- Pre-v0.4.0 graphs also stored actor IDs on `RunRecord.actor_id` in `_graph_runs.lance` / `_graph_run_actors.lance`. The Run state machine was removed in MR-771; those files are inert post-v0.4.0. The v2→v3 manifest migration sweeps any stale `__run__*` branches on first write-open (MR-770); the inert dataset bytes remain until a `delete_prefix` primitive lands.

docs/user/branches-commits.md

@@ -9,7 +9,7 @@ Lance supports branching at the dataset level: a branch is a named lineage of ve
 OmniGraph builds *graph branches* on top by branching every sub-table coherently:
 
 - `branch_create(name)` / `branch_create_from(target, name)` — disallowed name `main`; fails if branch exists; ensures the schema-apply lock is idle.
-- `branch_list()` — returns public branches, **filters internal** `__run__…` and `__schema_apply_lock__` prefixes.
+- `branch_list()` — returns public branches, **filters the internal** `__schema_apply_lock__` branch.
 - `branch_delete(name)` — refuses if there are descendants or active runs on the branch; cleans up owned per-branch fragments.
 - **Lazy forking**: a branch only forks a sub-table when that sub-table is first mutated on it. Pure-read branches share fragments with their source.
 - `sync_branch(branch)` — re-binds the in-memory handle to the latest head of the branch.
@@ -54,7 +54,7 @@ Notes:
 Filtered from `branch_list()` but visible to internals:
 
 - `__schema_apply_lock__` — serializes schema migrations.
-- `__run__<run-id>` — legacy from the pre-v0.4.0 Run state machine (removed in MR-771). The branch-name guard predicate `is_internal_run_branch` is kept as defense-in-depth so users cannot create a branch matching the legacy prefix; the filter will be removed once production legacy branches are swept (MR-770).
+- `__run__<run-id>` — legacy from the pre-v0.4.0 Run state machine (removed in MR-771). These are swept off `__manifest` on the first read-write open by the v2→v3 internal-schema migration (MR-770), and `__run__*` is no longer a reserved name. Known limitation: a pre-v0.4.0 graph opened **read-only** still surfaces any stale `__run__*` branch in `branch_list()` until its first read-write open (the migration is write-path-only, like all manifest migrations).
 
 ## L2 — Recovery audit trail
 

docs/user/constants.md

@@ -4,11 +4,11 @@
 |---|---|---|
 | `MANIFEST_DIR` | `__manifest` | `db/manifest/layout.rs` |
 | Commit graph dir | `_graph_commits.lance` | `db/commit_graph.rs` |
-| Run registry dir (legacy, removed MR-771) | `_graph_runs.lance` | inert post-v0.4.0; reclaimed by MR-770 |
-| Run branch prefix (legacy, removed MR-771) | `__run__` | filtered by `is_internal_run_branch` defense-in-depth |
+| Run registry dir (legacy, removed MR-771) | `_graph_runs.lance` | inert post-v0.4.0; bytes remain until a `delete_prefix` primitive lands |
+| Run branch prefix (legacy, removed MR-771/MR-770) | `__run__` | swept off `__manifest` by the v2→v3 migration; no longer a reserved name |
 | Schema apply lock | `__schema_apply_lock__` | `db/mod.rs` |
 | Manifest publisher retry budget | `PUBLISHER_RETRY_BUDGET = 5` | `db/manifest/publisher.rs` |
-| Internal manifest schema version | `INTERNAL_MANIFEST_SCHEMA_VERSION = 2` | `db/manifest/migrations.rs` |
+| Internal manifest schema version | `INTERNAL_MANIFEST_SCHEMA_VERSION = 3` | `db/manifest/migrations.rs` |
 | Merge stage batch | `MERGE_STAGE_BATCH_ROWS = 8192` | `exec/merge.rs` |
 | Maintenance concurrency | `OMNIGRAPH_MAINTENANCE_CONCURRENCY=8` | `db/omnigraph/optimize.rs` |
 | Graph index cache size | `8` (LRU) | `runtime_cache.rs` |

docs/user/storage.md

@@ -22,7 +22,7 @@ OmniGraph is **not** a single Lance dataset; it is a *graph* of datasets coordin
   - `edges/{fnv1a64-hex(edge_type_name)}` — one Lance dataset per edge type
   - `__manifest/` — the catalog of all sub-tables and their published versions
   - `_graph_commits.lance` / `_graph_commit_actors.lance` — the commit graph and its actor map
-  - (legacy `_graph_runs.lance` / `_graph_run_actors.lance` from pre-v0.4.0 graphs are inert; the run state machine was removed in MR-771 and these files are cleaned up via MR-770's production sweep)
+  - (legacy `_graph_runs.lance` / `_graph_run_actors.lance` from pre-v0.4.0 graphs are inert; the run state machine was removed in MR-771. The v2→v3 manifest migration sweeps stale `__run__*` branches on first write-open; the inert dataset bytes themselves remain until a `delete_prefix` storage primitive lands)
 - **Manifest row schema** (`object_id, object_type, location, metadata, base_objects, table_key, table_version, table_branch, row_count`):
   - `object_type` ∈ `table | table_version | table_tombstone`
   - `table_key` ∈ `node:<TypeName> | edge:<EdgeName>`
@@ -91,7 +91,7 @@ flowchart TB
 - **Graph root** is one directory (or S3 prefix). Everything below is part of one OmniGraph graph.
 - **`__manifest/`** is a Lance dataset whose rows describe which sub-table version is published at which graph-branch. Reading a snapshot starts here.
 - **`nodes/`** and **`edges/`** are sibling directories holding one Lance dataset per declared type. Names are `fnv1a64-hex` of the type name to keep paths fixed-length and case-safe.
-- **`_graph_commits.lance`** is an L2 dataset that records the graph-level commit DAG, with a paired `_graph_commit_actors.lance` for the actor map. (Pre-v0.4.0 graphs also have inert `_graph_runs.lance` / `_graph_run_actors.lance` from the removed Run state machine; MR-770 sweeps these in production.)
+- **`_graph_commits.lance`** is an L2 dataset that records the graph-level commit DAG, with a paired `_graph_commit_actors.lance` for the actor map. (Pre-v0.4.0 graphs also have inert `_graph_runs.lance` / `_graph_run_actors.lance` from the removed Run state machine; the v2→v3 migration sweeps their stale `__run__*` branches, and the dataset bytes are reclaimed once `delete_prefix` lands.)
 - **`_graph_commit_recoveries.lance`** — one row per recovery sweep action. Joined to `_graph_commits.lance` by `graph_commit_id`; the linked commit row carries `actor_id=omnigraph:recovery`. Operators correlate recoveries with the original mutations they rolled forward / back via this join. See `crates/omnigraph/src/db/recovery_audit.rs`.
 - **`__recovery/{ulid}.json`** — transient sidecar files written by the four migrated writers (`MutationStaging::finalize`, `schema_apply`, `branch_merge`, `ensure_indices`) before Phase B begins, deleted after Phase C succeeds. A sidecar persisting after process exit means the writer crashed in the Phase B → Phase C window; the next `Omnigraph::open` recovery sweep processes it. Steady-state directory is empty. See `crates/omnigraph/src/db/manifest/recovery.rs`.
 - **`_refs/branches/{name}.json`** is graph-level branch metadata — pointers from a branch name to the manifest version it heads.