Merge small Util files: OperatorSimp + CovDeriv consolidation

[Xinze-Li-Moqian]

Targets

Several Riemannian/Util/ files are very small and thematically adjacent:

File	LOC	Proposed
ConnectionLaplacianSimp.lean	41	merge into new OperatorSimp.lean
DivergenceSimp.lean	41	merge into new OperatorSimp.lean
CovDerivBridges.lean	52	merge into CovDerivSmoothness.lean

Result: 3 small files → 1 merged + 1 absorbed. Net -2 files.

Rationale

• ~40-50 LOC files don't justify their own file overhead (imports, namespace header, docstring).
• "Common Closure Principle" (Robert C. Martin) — operator simp lemmas coevolve, belong together.
• CLAUDE.md "Design is subtraction": prefer fewer files when content is small + thematically tight.

Cascade

Consumers of ConnectionLaplacianSimp / DivergenceSimp / CovDerivBridges update imports. Smallish (~5-10 sites).

Acceptance

• Build clean
• Shake baseline maintained
• Sorry count unchanged

Related

Depends on or conflicts with: #5 (Util sub-directory reorg) — if #5 lands first, this consolidation happens inside the Util/Simp/ and Util/CovDeriv/ sub-dirs.