surtgis v0.13.1

Patch release with a single, focused perf improvement: 5.66× faster
sky-view factor, which propagates to ~5× faster ambient_shade in
the relief composite pipeline. Bit-equivalent to within f64 rounding
(max delta 2.9e-15 over 363090 cells on dem_filled.tif); no
behavioural change for downstream callers.

Added

• surtgis_algorithms::terrain::sky_view_factor_fast(dem, params) — a
  hot-loop-optimised SVF that keeps the same SvfParams, output
  format, and NaN semantics as sky_view_factor. Three optimisations:

  • Pre-computed integer step offsets per direction. The Bresenham-
    style .round() that the reference implementation runs every
    iteration is hoisted out once per direction × step; the inner
    march becomes pure integer index arithmetic, which lets the
    autovectorisator keep up.
  • inv_dist[k] table for 1 / (k * cell_size) — replaces a
    division in the hot loop with one multiplication, shared across
    every cell × direction.
  • sin²(atan(t)) = t² / (1 + t²) algebraic identity at the
    direction summary — skips one atan and one sin per
    direction per cell (5.8 M trig calls saved on dem_filled.tif
    with 16 directions).

  Four new unit tests cover endpoint behaviour, pit-vs-open
  ordering, bit-equivalence against the reference SVF on a synthetic
  bumpy DEM, and zero-radius / zero-directions rejection.

• crates/algorithms/examples/bench_svf.rs — head-to-head benchmark
  used to validate the ≥3× acceptance bar before committing the swap.

Changed

• surtgis_relief::ambient_shade now calls sky_view_factor_fast.
  No public API change. Pipeline benchmark
  (crates/relief/examples/render_relief.rs on dem_filled.tif,
  radius=30):
  • ambient_shade: 6.36 s → 1.12 s (5.7×)
  • relief composite total: 6.6 s → 1.35 s (4.9×)