Derive a tighter compositional AAQDD error certificate

[maloleroy]

Motivation

The paper-critical implementation is sound, but the current universal certificate ||psi-m||_2 <= 1 + ||m||_2 is deliberately conservative and often too loose to establish useful accuracy.

Acceptance criteria

• Derive and document a gate-by-gate or merge-local sound error bound
• State the assumptions and give a reviewable proof
• Add deterministic and property-based containment tests
• Preserve 100% containment on the checked fidelity suite
• Demonstrate strictly tighter bounds on at least the layered and QAOA cases
• Regenerate the manuscript results and discuss remaining looseness

This is the highest-priority research follow-up after the initial manuscript.