refactor: retire the solid3d feature — the web build gets solids too

The feature existed for one reason: truck's mesh and boolean crates reach
`vtkio → xz2 → lzma-sys`, a C library that cannot cross-compile to wasm32. So
the web build dropped it, and with it the Model tab, solid tessellation and
ACIS import — all of which quietly did nothing in the browser.

cadkernel is pure Rust and has no C dependency, so none of that holds any
more. The feature gated exactly two things by the end, `acis_export` and the
save-path sync that calls it, and both are as portable as everything around
them. Removing it means the web build makes primitives, runs booleans, draws
ACIS solids read from a file, and writes them back out as exact geometry — the
same as the desktop build.

Verified against all three: native, `--no-default-features`, and an actual
`wasm32-unknown-unknown` check rather than an assumption about one.

The docs claimed otherwise in four places and are corrected. `tessellation.md`
in particular was organised around whether an entity "goes through truck",
which is no longer a question anything can be asked — the three paths are now
a kernel B-rep mesh, a curve sampled through `entities::curve`, or geometry
emitted directly, and the per-entity table is updated to match.

`index.html` no longer passes `data-cargo-no-default-features`; there are no
features left to turn off.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/pages.yml

@@ -50,10 +50,8 @@ jobs:
         with:
           tool: wasm-bindgen@${{ steps.wasm-bindgen-version.outputs.version }}
 
-      # The rust <link> in index.html sets `data-cargo-no-default-features`,
-      # so this drops the `solid3d` feature (truck-meshalgo → lzma-sys C deps
-      # that can't cross-compile to wasm). `--public-url` matches the project
-      # page sub-path: https://<user>.github.io/OpenCADStudio/.
+      # `--public-url` matches the project page sub-path:
+      # https://<user>.github.io/OpenCADStudio/.
       - name: Build web bundle
         run: trunk build --release --public-url /OpenCADStudio/