feat(costmap): generic grid projection primitives and typed layer access

[MJohnson459]

Add the substrate needed to feed an arbitrary, caller-owned Grid2d (terrain class, learned traversability, instance ids, ...) into the u8 master costmap as a first-class layer, and to read it back for control — without putting any semantic/learned types in the crate itself.

• project_into + MergePolicy (src/costmap/project.rs): the keystone generic projection of Grid2d -> master via a T -> Option closure. merge_overwrite/merge_max/merge_max_keep_unknown now delegate to it (one codepath; signatures unchanged).
• Typed layer access (src/costmap/layered.rs): Layer: Any, LayerId, and add_layer returning a LayerId plus layer/layer_mut downcast accessors, so a layer can own its grid and the caller ingests/queries through &mut LayeredCostmap (no Arc/RwLock). Also makes every layer's internal grid inspectable.
• ProjectionLayer (src/layers/projection.rs): an owned-source layer (nav2 CostmapLayer generalized over T + a projection closure). For rolling windows it re-centres the source in update_bounds so producers that fill during update_costs see an already-centred grid.
• cost_from_unit/cost_from_range (src/types/cost.rs) and Grid2d::layout_matches (src/grid/grid2d.rs).
• examples/learned_traversability.rs: end-to-end rolling-window demo (mock model -> ProjectionLayer -> inflation -> read-back control decision). SimLidarLayer in local_costmap_lidar.rs refactored to compose ProjectionLayer, validating the abstraction against the existing pattern.

Tests cover projection policies, layout_matches, cost conversion, typed access, update_bounds re-centring/extent, the rolling end-to-end path, and projection seeding inflation.