Add OpenStreetMap base layer with Carto tile overlay

- Fetch Carto Voyager no-label base tiles and label-only tiles separately,
  cached under cache/osm_tiles/{z}/ and cache/osm_tiles/labels/{z}/
- Rasterize both into the orthographic projection via exact inverse projection
  (new unproject/pixel_to_geo methods on OrthoProjection)
- Render pipeline: basemap → cloud blend → label overlay (labels above clouds)
- Bilinear interpolation for base tiles; premultiplied-alpha bilinear for label
  tiles to prevent dark-fringe artifacts at text edges
- Dynamic zoom selection (floor-based) from actual geographic bounding box
- Fix horizontal squish: derive HALF_W_DEG from pixel aspect ratio so
  degrees-per-pixel is equal on both axes
- Add --no-basemap flag to skip tile fetching for offline/fast use
- Remove hardcoded city markers/labels when tile label overlay is present

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.rs

@@ -2,6 +2,7 @@ mod download;
 mod grib;
 mod projection;
 mod render;
+mod tiles;
 
 use anyhow::{bail, Context, Result};
 use chrono::{DateTime, Datelike, Timelike, Utc};
@@ -29,6 +30,10 @@ struct Args {
     /// Number of forecast hours to download and render (0–48)
     #[arg(value_name = "HOURS")]
     prediction_hours: u32,
+
+    /// Disable OpenStreetMap base layer (use flat green background)
+    #[arg(long)]
+    no_basemap: bool,
 }
 
 fn main() -> Result<()> {
@@ -125,6 +130,53 @@ fn main() -> Result<()> {
     })?;
     cloud_data.sort_unstable_by_key(|(step, _)| *step);
 
+    // Fetch and rasterize OSM basemap (once, reused for all frames).
+    let basemap = if args.no_basemap {
+        None
+    } else {
+        let proj = projection::OrthoProjection::new(
+            render::CENTER_LAT,
+            render::CENTER_LON,
+            render::HALF_W_DEG,
+            render::HALF_H_DEG,
+        );
+        let tile_cache = tiles::fetch_tiles(
+            &proj,
+            render::MAP_W,
+            render::MAP_H,
+            &PathBuf::from(CACHE_DIR),
+        )?;
+        Some(tiles::rasterize_basemap(
+            &proj,
+            render::MAP_W,
+            render::MAP_H,
+            &tile_cache,
+        ))
+    };
+
+    let labels = if args.no_basemap {
+        None
+    } else {
+        let proj = projection::OrthoProjection::new(
+            render::CENTER_LAT,
+            render::CENTER_LON,
+            render::HALF_W_DEG,
+            render::HALF_H_DEG,
+        );
+        let label_cache = tiles::fetch_label_tiles(
+            &proj,
+            render::MAP_W,
+            render::MAP_H,
+            &PathBuf::from(CACHE_DIR),
+        )?;
+        Some(tiles::rasterize_labels(
+            &proj,
+            render::MAP_W,
+            render::MAP_H,
+            &label_cache,
+        ))
+    };
+
     eprintln!("Rendering {} frame(s)...", args.prediction_hours);
     let mut output_paths = Vec::new();
 
@@ -156,7 +208,7 @@ fn main() -> Result<()> {
         let out_name = format!("clct_{:06}.png", step + 1);
         let out_path = cache_dir.join(&out_name);
 
-        render::render_frame(&out_path, &lats, &lons, &cloud, &title)
+        render::render_frame(&out_path, &lats, &lons, &cloud, &title, basemap.as_deref(), labels.as_deref())
             .with_context(|| format!("Rendering frame {}", step))?;
 
         eprintln!("  [{:02}/{:02}] {}", step + 1, args.prediction_hours, out_name);