Planar all-quad meshing: size fields, TFI mapped meshes, free quad mesher (M5 base) (#18)

* Planar all-quad meshing: size fields, TFI mapped meshes, free quad mesher, planar quad validity

- modelcraft_mesh::size: SizeField trait, division counts, graded polyline division, per-loop parity
- modelcraft_mesh::quad: transfinite (Gordon-Hall) mapped meshes of four-sided regions; free
  all-quad meshes of polygons with holes on prescribed boundary nodes (half-resolution CDT with
  Ruppert refinement, Lee-Lo recombination, uniform split, smart Laplacian plus
  Freitag-Plassmann untangling), exact validity gate on every element
- modelcraft_mesh::quality: Robinson aspect ratio and skew for quads, exact planar quad validity

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* quad mesher: true-boundary guard for Steiner points, untangling by corner area, parity retry, recombination quality floor; I-beam and perforated plate tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: quad meshing papers (B7), roadmap M5 and M2, architecture pipeline step

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* quad mesher: simplify (shared corner and validity helpers, vertex rotation helper, guard decoupled from the CDT, verify inside the CDT, non-allocating final gate)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/mesh/src/predicates/mod.rs

@@ -117,6 +117,13 @@ pub fn is_admissible(x: f64) -> bool {
     x == 0.0 || (x.abs() >= MIN_MAGNITUDE && x.abs() <= MAX_MAGNITUDE)
 }
 
+/// `x` with a magnitude below [`MIN_MAGNITUDE`] rounded to zero (tessellation and construction
+/// noise); other values unchanged. The result is admissible when `|x| <= MAX_MAGNITUDE`.
+#[inline]
+pub fn snap_to_domain(x: f64) -> f64 {
+    if x.abs() < MIN_MAGNITUDE { 0.0 } else { x }
+}
+
 fn check<const N: usize>(c: [f64; N]) -> Result<[f64; N]> {
     match c.iter().find(|x| !is_admissible(**x)) {
         None => Ok(c),