Engine: the meshing commands pass the design's edge seeds to the meshers

mesh.tet.generate, mesh.hex.*, mesh.hex.blocks and mesh.cells give every
seed on the open design to their meshers, which match them to their own
curves by position, so a seed on an edge two cells share holds in both. The
tet reply gives, per seed, the mesh edges along it. Docs and the roadmap
say what is honoured where.

Fixes #230.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/mcp.md

@@ -153,7 +153,13 @@ edges of partition cells matching counts.
 2. `execute` `mesh.seed.edge {edges: [{body, edge}], count | size, bias?: {ratio, toward:
    start|end|both}, constraint?: exact|min|free}`. `exact` keeps the count, `min` keeps at least
    it, `free` is a goal. The reply gives the count each edge gets.
-3. `mesh.seed.list` lists the seeds and the edges they reach (`stale` when the design has no such
+3. Mesh: the hex meshers (`hex`, `hex_sweep`, `hex_mapped`, `hex_blocks`, `cells`) keep exact counts
+   through their interval assignment and refuse, naming the seeds, when two exact counts cannot
+   both hold (opposite sides of a mapped face, the layers between two levels); the tet mesher
+   divides the edge at the seed's nodes and grades its size field from them (its reply's
+   `seeds` gives the mesh edges along each). A seed on an edge two partition cells share holds
+   in both, so their meshes still conform.
+4. `mesh.seed.list` lists the seeds and the edges they reach (`stale` when the design has no such
    edge); `mesh.seed.clear {edges?}` removes them. Seeds are saved with the model.
 
 ## Limits