refactor(M03): extract M2 build batch planner

docs/modules/m2-placement-grouper.md

@@ -153,8 +153,8 @@ continue to own task/build/cache observability.
 
 ## Extension points
 
-- A later package may extract the M2 build-job state machine behind explicit
-  budgets while retaining this pure grouping input.
+- `M2BuildBatchPlanner` now owns pure batch sizing/cursor progression; the
+  remaining build-job state machine stays behind explicit loader budgets.
 - Spatial cells require measured culling evidence and are outside this contract.
 
 ## Capability status
@@ -179,11 +179,13 @@ continue to own task/build/cache observability.
 |---|---|
 | `src/render/m2/m2_placement_grouper.gd` | Validation, normalization and grouping |
 | `src/render/m2/m2_placement_transform_resolver.gd` | Basis and origin rules |
+| `src/render/m2/m2_build_batch_planner.gd` | Downstream batch sizing and cursor planning |
 | `src/scenes/streaming/streaming_world_loader.gd` | Worker/result/build lifecycle and resources |
 | `src/tools/verify_m2_placement_grouper.gd` | Contract, dependency and timing regression |
 
 ## Related decisions and references
 
+- [`m2-build-batch-planner.md`](m2-build-batch-planner.md)
 - [`m2-placement-transform-resolver.md`](m2-placement-transform-resolver.md)
 - [`m2-unique-placement-registry.md`](m2-unique-placement-registry.md)
 - [`world-renderer.md`](world-renderer.md)