Add runout avoidance

[justmytwospence]

Plan: Add Avalanche Runout Zone Avoidance

Compute runout zones eagerly when aspects are calculated, using the 10° beta point (gradient ≈ 0.176 rise/run) from avalanche literature. Runout zones will be avoided during pathfinding and visualized with a distinct amber hue.

Algorithm Overview

Runout Zone Computation (in Rust):

1. For each pixel where gradient >= 0.176 AND aspect ∈ excluded_aspects: mark as source zone
2. For each source zone pixel, perform steepest-descent flow to propagate danger downhill:
   • Look at all 8 neighbors, find the one with lowest elevation
   • If that neighbor's gradient is still ≥ 0.176 (10°), mark it as runout and continue flowing from there
   • Stop when slope drops below 10° (the beta point where debris typically stops)
3. Output: binary raster where 1 = runout zone, 0 = safe

Steps

1. Add compute_runout_zones function in azimuth.rs: Accept excluded_aspects: Vec<String>, elevations: &Vec<Vec<f64>>, azimuths: &Vec<Vec<f64>>, gradients: &Vec<Vec<f64>>. Iterate all pixels; for each source cell (steep + excluded aspect), do BFS/flood-fill following steepest descent until gradient < 0.176. Return Vec<Vec<u8>> runout mask.

2. Extend AzimuthResult and compute_azimuths in azimuth.rs: Add excluded_aspects: &[String] parameter to compute_azimuths. Call compute_runout_zones after computing azimuths/gradients. Add runout_zones: Vec<u8> field to AzimuthResult (serialized as GeoTIFF).

3. Update TypeScript WASM wrapper in pathfinder-wasm.ts: Pass excludedAspects array to compute_azimuths, receive and return runout_zones buffer.

4. Thread runout through find-path-button.tsx: Pass excludedAspects when calling aspect computation, forward runout_zones to handleSetAspectRaster callback.

5. Add runout exclusion in find_path.rs: Accept runout_zones buffer, parse as raster, add check in successors closure: if runout_zones[ny][nx] > 0 { break 'neighbors; } alongside existing aspect check.

6. Update handleSetAspectRaster in page.tsx: Accept third runout_zones: Uint8Array parameter, parse with parseGeoraster, merge as third band into aspectRaster state.

7. Add runout visualization in leaflet-raster-layer.tsx: Read third band (values[2]) from aspectRaster. For pixels where runout > 0, render with amber/orange color (rgba(245, 158, 11, 0.5)) distinct from the red excluded-aspect shading.

Further Considerations

1. Caching invalidation: Runout depends on excludedAspects, so changing aspects requires recomputing runout. Should we cache multiple runout configurations, or always recompute? Recommend: always recompute—it's fast and aspects rarely change mid-session

2. Steep terrain without excluded aspects: Currently runout only flows from excluded-aspect slopes. Should all steep terrain generate runout regardless of aspect selection? Recommend: only excluded aspects, since user explicitly chose what to avoid

3. Flow algorithm choice: BFS flood-fill vs. following single steepest-descent path. BFS captures wider runout fans but is slower. Recommend: start with single steepest-descent path per source cell for performance, can expand later