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>

index.html

@@ -39,9 +39,10 @@
       }
       @keyframes ocs-spin { to { transform: rotate(360deg); } }
     </style>
-    <!-- Trunk builds the wasm32 binary without the `solid3d` feature (no 3-D
-         solid kernel / vtkio C deps on the web). See issue #45. -->
-    <link data-trunk rel="rust" data-bin="OpenCADStudio" data-cargo-no-default-features />
+    <!-- Trunk builds the wasm32 binary. The crate has no Cargo features:
+         the geometry kernel is pure Rust, so the web build gets the same
+         solid modelling the native one does. See issue #45. -->
+    <link data-trunk rel="rust" data-bin="OpenCADStudio" />
     <!-- Per-script Noto subsets, fetched lazily at runtime (one alphabet per
          file) so CAD text renders non-Latin scripts on the web. (#141) -->
     <link data-trunk rel="copy-dir" href="web/fonts" />