feat(web): render hatches on WebGL2 via a texture-backed renderer

WebGL2 has no vertex/fragment storage buffers, so the batched storage-buffer
hatch renderer (hatch_gpu) is disabled on wasm and real hatch fills never
rendered on the web build — the pick highlighted the boundary and the command
then appeared to cancel (issue #204).

Add a wasm-only per-hatch renderer (hatch_web_gpu + hatch_web.wgsl) that packs
the variable-length boundary / family / dash arrays into one RGBA32F data
texture read with textureLoad, instead of the fixed-size uniforms of the
wipeout path. This removes the MAX_FAMILIES (16) / MAX_HATCH_BOUNDARY_VERTS
(1024) / MAX_DASHES (128) caps, so every hatch — solid, gradient, and
arbitrarily complex line patterns — renders on the web. The fill algorithm
(in_polygon, per-family line/dash/dot, gradient, LOD, double-single vertex
transform) is identical to the desktop path; only the array ACCESS changes.
Pattern phase is anchored at world_origin with the boundary stored raw, and
scale / gradient-range clamped, to match the desktop batched renderer exactly.

Native (hatch_gpu + hatch.wgsl) and the wipeout mask renderer are untouched;
the new module and pipeline are gated on target_arch = "wasm32".

Also silence the resulting (and pre-existing) wasm-only dead-code warnings for
native-only paths so both the native and web builds compile clean.

Closes #204.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/io/thumbnail.rs

@@ -123,6 +123,7 @@ fn to_rgb(c: [f32; 4]) -> [u8; 3] {
 /// generic icon. Backs the hidden `--dwg-thumbnail` mode the installed
 /// freedesktop `.thumbnailer` invokes. Extraction lives in the shared
 /// [`dwg_thumbnailer`] core (also used by the Windows/macOS handlers).
+#[cfg_attr(target_arch = "wasm32", allow(dead_code))]
 pub fn extract_to_png(input: &std::path::Path, output: &std::path::Path, size: u32) -> bool {
     match dwg_thumbnailer::extract(input, size) {
         Some(mut img) => {