* fix(cognition): add explicit surface resolution policy
* test(cognition): cover explicit surface resolution policy
* fix(cognition): route pipeline surfaces through resolver
* fix(cognition): address PR #76 review comments
- hoist `_is_useful_surface` import from inside `run()` to module top
- call `_render_walk_surface` / `_render_compose_surface` via the class
name (both are @staticmethod) for consistency with the existing
`_fold_*_into_surface` helpers
- drop redundant `realized_surface` truthiness check in
`resolve_surface` — `realizer_useful` already excludes empty /
placeholder surfaces via `_is_useful_surface`
Tests: tests/test_surface_resolution.py + tests/test_cognitive_turn_pipeline.py
green (16 passed); cognition suite 120/1s, smoke suite 67/0.
