improve visibility checker and add refiner

DOCS.md

@@ -19,6 +19,7 @@ The `AStarContext` stores the configuration and persistent state for the A* sear
 | `sbend_penalty`       | `float`       | 500.0              | Flat cost added for every S-bend.                                                     |
 | `bend_collision_type` | `str`         | `"arc"`            | Collision model for bends: `"arc"`, `"bbox"`, or `"clipped_bbox"`.                    |
 | `bend_clip_margin`    | `float`       | 10.0               | Extra space (µm) around the waveguide for clipped models.                             |
+| `visibility_guidance` | `str`         | `"tangent_corner"` | Visibility-driven straight candidate mode: `"off"`, `"exact_corner"`, or `"tangent_corner"`. |
 
 ## 2. AStarMetrics
 
@@ -86,6 +87,12 @@ If the router produces many small bends instead of a long straight line:
 2.  Ensure `straight_lengths` includes larger values like `25.0` or `100.0`.
 3.  Decrease `greedy_h_weight` closer to `1.0`.
 
+### Visibility Guidance
+The router can bias straight stop points using static obstacle corners.
+- **`"tangent_corner"`**: Default. Proposes straight lengths that set up a clean tangent bend around nearby visible corners. This helps obstacle-dense layouts more than open space.
+- **`"exact_corner"`**: Only uses precomputed corner-to-corner visibility when the current search state already lands on an obstacle corner.
+- **`"off"`**: Disables visibility-derived straight candidates entirely.
+
 ### Handling Congestion
 In multi-net designs, if nets are overlapping:
 1.  Increase `congestion_penalty` in `CostEvaluator`.