surtgis v0.10.4

Patch release that adds in-tree PNG output. Pre-M1 for the
surtgis-relief crate (see SPEC_SURTGIS_RELIEF.md); doing this in
surtgis-colormap rather than surtgis-relief so every crate that
produces an RGBA buffer (curvature previews, hypsometric maps, fluvial
GeoJSON-paired rasters, future relief composites) can save PNG without
pulling its own dependency.

Added

• RgbaImage struct in surtgis_colormap::encode with the row-major
  RGBA pixel layout that raster_to_rgba already produces. Methods:
  from_rgba, from_intensity (single-channel [0, 1] raster →
  greyscale), over (alpha-over composite), multiply (multiply blend
  for shadows on a colored base).
• RgbaImage::to_png_bytes() and RgbaImage::save_png(path) for
  native PNG output. Standalone helpers rgba_to_png_bytes(width, height, &[u8]) and save_png(path, …) are also re-exported at the
  crate root.
• EncodeError typed via thiserror; includes shape-mismatch,
  image::ImageError, and std::io::Error variants.

Notes

• All PNG paths are gated on cfg(not(target_arch = "wasm32")). The
  image crate is added as a workspace-target dependency under the
  same gate, so WASM builds stay lightweight. On WASM, consumers are
  expected to pass the raw RGBA buffer back to JS, where the canvas or
  Blob path encodes the image.
• No algorithm changes elsewhere; surtgis-core, surtgis-algorithms,
  surtgis-parallel, surtgis-cloud, surtgis (CLI) ship unchanged
  but bump in lockstep with the workspace version.