Add the substrate needed to feed an arbitrary, caller-owned Grid2d<T>
(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<T> -> master via a T -> Option<u8> 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<T> (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<u8>, 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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>