Native format: document.save can write path coordinates at a fixed precision

Native files store every anchor and handle as a full double, such as
3.4876474632797176, so a file of traced art is larger than it needs
to be.

document.save, file.saveAs, file.saveCopy and file.saveAsTemplate take
precision (0 to 9) for native files. The save rounds the anchors and
handles of path objects, Live Corners' uncut outlines, blend spines,
envelope outlines, Intertwine areas and brush art to that many decimal
places, in the layers, symbols, graph designs, patterns and opacity
masks, and in the art that placed documents become in files for older
versions. Colors, transforms, opacity and type are written exactly.
Without precision the file is written as before, and Save does not
remember the option. A .ai file, saved or exported, ignores precision.
The VectorCraft Options dialog shows no row for it.

A save given precision beside the path for a format that does not read
it (SVG, PDF, .ai and .ait) says so in its warnings, as it does for
compress, version and preview. MCP save_file passes precision on to the
save as it passes the other native options, and its schema, description
and docs/mcp.md list it.

crates/format/src/lib.rs

@@ -229,11 +229,26 @@ pub struct SaveOptions {
     pub profiles: BTreeMap<String, Vec<u8>>,
     /// Create PDF-Compatible File: a PDF of the document to carry.
     pub pdf: Option<Vec<u8>>,
+    /// Decimal places to round path anchors and handles to ([`Document::round_geometry`]), at
+    /// most [`MAX_PRECISION`] (a larger value rounds to that many); `None` writes them exactly.
+    pub precision: Option<u8>,
 }
 
+/// The most decimal places [`SaveOptions::precision`] rounds path geometry to.
+pub const MAX_PRECISION: u8 = 9;
+
 impl Default for SaveOptions {
     fn default() -> Self {
-        Self { pretty: false, compress: false, version: VERSION, preview: None, include_linked: false, profiles: BTreeMap::new(), pdf: None }
+        Self {
+            pretty: false,
+            compress: false,
+            version: VERSION,
+            preview: None,
+            include_linked: false,
+            profiles: BTreeMap::new(),
+            pdf: None,
+            precision: None,
+        }
     }
 }
 
@@ -299,7 +314,8 @@ struct Body {
 }
 
 /// `doc` as [`save_to`] writes it with `o`: without editing-mode working copies, with placed
-/// documents as their art for older versions, and its images taken out.
+/// documents as their art for older versions, path geometry rounded to `o.precision`, and its
+/// images taken out.
 fn body(doc: &Document, o: &SaveOptions) -> Result<Body, FormatError> {
     let version = o.version;
     let mut d = doc.without_edit_modes().into_owned();
@@ -311,6 +327,10 @@ fn body(doc: &Document, o: &SaveOptions) -> Result<Body, FormatError> {
     } else {
         d.drop_placed_resources();
     }
+    // The art placed documents became for older versions is rounded too.
+    if let Some(decimals) = o.precision {
+        d.round_geometry(decimals.min(MAX_PRECISION));
+    }
     let linked = d.linked_only_images();
     // The blobs go in the file's `images` (only those the document uses).
     let blobs = std::mem::take(&mut d.images);