more s-bend fixes and add docs

DOCS.md

@@ -0,0 +1,53 @@
+# Inire Configuration & API Documentation
+
+This document describes the user-tunable parameters for the `inire` auto-router.
+
+## 1. AStarRouter Parameters
+
+The `AStarRouter` is the core pathfinding engine. It can be configured directly through its constructor.
+
+| Parameter | Type | Default | Description |
+| :--- | :--- | :--- | :--- |
+| `node_limit` | `int` | 1,000,000 | Maximum number of states to explore per net. Increase for very complex paths. |
+| `straight_lengths` | `list[float]` | `[1.0, 5.0, 25.0]` | Discrete step sizes for straight waveguides (µm). Larger steps speed up search in open space. |
+| `bend_radii` | `list[float]` | `[10.0]` | Available radii for 90-degree turns (µm). Multiple values allow the router to pick the best fit. |
+| `sbend_offsets` | `list[float]` | `[-5, -2, 2, 5]` | Lateral offsets for parametric S-bends (µm). |
+| `sbend_radii` | `list[float]` | `[10.0]` | Available radii for S-bends (µm). |
+| `snap_to_target_dist`| `float` | 20.0 | Distance (µm) at which the router attempts an exact bridge to the target port. |
+| `bend_penalty` | `float` | 50.0 | Flat cost added for every 90-degree bend. Higher values favor straight lines. |
+| `sbend_penalty` | `float` | 100.0 | Flat cost added for every S-bend. Usually higher than `bend_penalty`. |
+| `bend_collision_type`| `str` | `"arc"` | Collision model for bends: `"arc"`, `"bbox"`, or `"clipped_bbox"`. |
+| `bend_clip_margin` | `float` | 10.0 | Margin (µm) for the `"clipped_bbox"` collision model. |
+
+### Bend Collision Models
+*   `"arc"`: High-fidelity model following the exact curved waveguide geometry.
+*   `"bbox"`: Conservative model using the axis-aligned bounding box of the bend. Fast but blocks more space.
+*   `"clipped_bbox"`: A middle ground that uses the bounding box but clips corners that are far from the waveguide.
+
+---
+
+## 2. CostEvaluator Parameters
+
+The `CostEvaluator` defines the "goodness" of a path.
+
+| Parameter | Type | Default | Description |
+| :--- | :--- | :--- | :--- |
+| `unit_length_cost` | `float` | 1.0 | Cost per µm of wire length. |
+| `greedy_h_weight` | `float` | 1.1 | Heuristic weight. `1.0` is optimal; higher values (e.g., `1.5`) are faster but may produce longer paths. |
+| `congestion_penalty`| `float` | 10,000.0 | Multiplier for overlaps in the multi-net Negotiated Congestion loop. |
+
+---
+
+## 3. CollisionEngine Parameters
+
+| Parameter | Type | Default | Description |
+| :--- | :--- | :--- | :--- |
+| `clearance` | `float` | (Required) | Minimum required distance between any two waveguides or obstacles (µm). |
+| `safety_zone_radius`| `float` | 0.0021 | Radius (µm) around ports where collisions are ignored to allow PDK boundary incidence. |
+
+---
+
+## 4. Physical Units & Precision
+-   **Coordinates**: Micrometers (µm).
+-   **Grid Snapping**: The router internally operates on a **1nm** grid for final ports and a **1µm** lattice for expansion moves.
+-   **Search Space**: Assumptions are optimized for design areas up to **20mm x 20mm**.