Save as mesh: one dialog for STL and 3MF, with Send to slicer (#1272)

One docked Save as mesh dialog replaces Export STL / 3MF / 3MF with face tags, with an optional Send to slicer that lists the slicers installed on this machine. Engine: file.save_mesh and file.slicers; the Convert Mesh dialog goes (mesh.convert and the MeshConvert feature stay, so existing designs open). Strings in all catalogs; screenshots in the PR. Checked on main + this branch: cargo check --workspace, the engine slicer/mesh/file tests, the UI catalog and menu tests and scenario s14 (the one local colour-token failure is epaint's TexturesDelta panic, which main shows on this machine too).

docs/book/src/guide/files.md

@@ -16,21 +16,16 @@ Opening a STEP or IGES file creates a design with the file's bodies as a base fe
 inserting one adds them to the current design. Later features build on them. Mesh bodies render,
 measure, move and export, but solid features need B-rep bodies.
 
-For closed mechanical meshes with a constant cross-section, use **Mesh → Modify →
-Convert Mesh**. Select the mesh body and set the fitting tolerance. The conversion
-rebuilds planar faces and circular through-walls, replacing the mesh with a solid
-that accepts solid modeling operations. The imported triangles remain embedded
-upstream: changing their scale, undoing the conversion or reopening the design
-rebuilds the result through the timeline. Double-click the conversion to edit its
-tolerance or source body.
+**File → Save as mesh…** (also on the Mesh tab under Modify, and in the right-click menu of a
+body) saves the selected bodies as STL, 3MF or 3MF with face tags (below). With **Send to slicer** ticked, it writes the file to
+a temporary folder and opens it in the chosen slicer instead of asking for a file name. The slicer
+list shows the slicers found on the computer with their icons; **Custom…** adds any other program
+to the top of the list (kept between runs; hover an entry you added and press Delete to remove it).
 
-The tolerance bounds circular reconstruction against the original chords. Coarse
-polygons remain polygons when rounding would exceed it. Open/nonmanifold meshes,
-tapers, stepped parts and organic scans are refused by this first reconstruction
-method; it does not silently substitute a faceted solid. Conversion currently
-accepts at most 50,000 triangles and a bounded number of boundary segments.
+Closed mechanical meshes with a constant cross-section can be rebuilt as solids with the
+`mesh.convert` command (command palette, scripts and the control channel).
 
-**File → Export 3MF with Face Tags** (`file.export` with `format: "3mf-tagged"`) writes an
+**File → Save as mesh…** with the format **3MF with face tags** (`file.export` or `file.save_mesh` with `format: "3mf-tagged"`) writes an
 ordinary 3MF in which every triangle also names the B-rep face it lies on and every coordinate
 is the tessellation's own double-precision value. Meshing and analysis tools can use the tags to
 keep a model's faces (for loads, boundary conditions or feature edges); other 3MF readers ignore

docs/formats/3mf-face-tags.md

@@ -7,7 +7,7 @@ A tessellated body loses its faces: a plain mesh says where the surface is, not
 each triangle came from. Downstream tools that mesh or analyse a part need that back. They apply
 loads and boundary conditions to faces, keep face boundaries as feature edges, and group
 results by face. SolidCraft therefore exports a 3MF in which every triangle carries the index of
-the B-rep face it lies on (**File → Export 3MF with Face Tags**, or `file.export` with
+the B-rep face it lies on (**File → Save as mesh…** (format **3MF with face tags**), or `file.export` with
 `format: "3mf-tagged"`).
 
 ## Why 3MF