[pbrubeck]

Summary

 MeshGeometry equality
- UFL ID plus coordinate element
+ object identity
+ matching identity hash for Firedrake and UFL cache keys

 Submesh(parent, comm=COMM_SELF)
+ compares distinct from parent
+ assigns across mesh communicators correctly

Evidence

• Before: test_submesh_comm_self_distinct_from_parent[nprocs=1] failed because the parent mesh and submesh both had UFL ID 1 and compared equal.
• After: the regression test passes at nprocs=1 and nprocs=3; make srclint also passes.

Merge Danger

Door: two-way

The change is localized to mesh equality and one submesh communicator regression test. Identity hashing preserves mesh use as a cache key after overriding equality.

Blast Radius: narrow

Contributions assisted by Codex.