mesh: a symmetric plan copies one corner onto the other (#474, #491)

With symmetry, when the fundamental cell's two ends along another in-plane
axis are alike (layout planes and surface mirror-symmetric), the cell is
cut midway into the next block sheet and at its image; the low corner is
meshed with its blocks, mirrored onto the high one, and the middle meshed
with both cut faces fixed (#490). A plan cell's cut faces another cell's
mesh covers are taken as given. corners: false opts out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

apps/fieldcraft-cli/tests/update.rs

@@ -12,7 +12,10 @@ fn cli(host: &str, args: &[&str]) -> std::process::Output {
 #[test]
 fn update_check_reports_the_latest_release() {
     let server = Server::start().unwrap();
-    server.release("latest", &json!({"tag_name": "v99.0.0", "html_url": "https://github.com/tangency-labs/fieldcraft/releases/tag/v99.0.0", "assets": []}));
+    server.release(
+        "latest",
+        &json!({"tag_name": "v99.0.0", "html_url": "https://github.com/tangency-labs/fieldcraft/releases/tag/v99.0.0", "assets": []}),
+    );
     let out = cli(&server.url(), &["update", "--check"]);
     assert!(out.status.success(), "{out:?}");
     let report: Value = serde_json::from_slice(&out.stdout).unwrap();