Export 3MF with B-rep face tags (#834)

3MF export carries each triangle's B-rep face tag.

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

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+# 3MF with B-rep face tags
+
+Version 1 (2026-10). Namespace `https://github.com/bherbruck/solidcraft/formats/3mf-face-tags/2026/10`,
+prefix `ft`.
+
+A tessellated body loses its faces: a plain mesh says where the surface is, not which B-rep face
+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
+`format: "3mf-tagged"`).
+
+## Why 3MF
+
+The file is an ordinary **3MF package** (3MF Consortium, *3D Manufacturing Format Core
+Specification* 1.3). The tags are one attribute in a namespace of SolidCraft's. The Core
+specification allows that attribute on `<triangle>` and `<object>`, and every consumer ignores it
+because the namespace is not listed in `requiredextensions`. Slicers, viewers and SolidCraft's
+own mesh import open the file as plain 3MF. A tool that knows the namespace gets the faces too.
+A purpose-built binary format would have needed its own specification and readers, and would
+open nowhere else. Decimal coordinates lose nothing: written in shortest round-trip form, every
+`f64` reads back bit for bit.
+
+## Example
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<model unit="millimeter" xml:lang="en-US"
+       xmlns="http://schemas.microsoft.com/3dmanufacturing/core/2015/02"
+       xmlns:ft="https://github.com/bherbruck/solidcraft/formats/3mf-face-tags/2026/10">
+ <metadata name="Application">SolidCraft</metadata>
+ <resources>
+  <object id="2" type="model" name="Body1" ft:faces="7">
+   <mesh>
+    <vertices>
+     <vertex x="0" y="0" z="0"/>
+     ...
+    </vertices>
+    <triangles>
+     <triangle v1="0" v2="2" v3="1" ft:face="0"/>
+     ...
+    </triangles>
+   </mesh>
+  </object>
+ </resources>
+ <build>
+  <item objectid="2"/>
+ </build>
+</model>
+```
+
+## Attributes
+
+| Attribute | On | Value | Meaning |
+|---|---|---|---|
+| `ft:faces` | `<object>` | integer ≥ 1 | Number of B-rep faces of the body. Every `ft:face` of the object is below it. Faces without triangles still count. |
+| `ft:face` | `<triangle>` | integer ≥ 0 | The B-rep face (0-based, in the body's own face order) the triangle lies on. |
+
+An object's triangles are either all tagged or none. A reader treats an object without tags as a
+single face (tag 0).
+
+## What SolidCraft writes
+
+- `unit="millimeter"`, Z up. There is one `<object type="model">` with a `<mesh>` per body,
+  named after the body, and one `<item>` per object, with no transform and no components.
+- The tessellation is the export mesh (chord tolerance 2·10⁻⁴ of the body size). It is welded
+  watertight: vertices within 10⁻⁶ of the body size are merged and T-junctions between faces are
+  split. Each piece of a split triangle keeps its face. Triangles are counter-clockwise seen from
+  outside, and degenerate triangles are dropped.
+- Coordinates are the tessellation's own `f64` values in shortest round-trip form, and vertices
+  are shared by exact value.
+- Colours use 3MF's own `basematerials` (`pid`/`p1`), as in the plain 3MF export, independent of
+  the tags.
+- The bytes are deterministic: fixed part order, fixed zip timestamps, no hash-ordered output.
+
+## Versions
+
+A later version gets a new namespace (`…/3mf-face-tags/<year>/<month>`). A reader accepts the
+versions it knows and treats an unknown face-tag namespace as untagged.