Contracts: Concurrent `register` vs `max_cap` race / off-by-one audit

[joelpeace48-cell]

Context

register enforces get_max_cap/get_participant_count. Under high concurrent submission near the
cap, off-by-one or read-modify-write ordering can let participant count exceed the cap or reject the
exact-Nth registrant incorrectly.

Scope

• Audit the count increment vs cap check ordering for atomicity within a single invocation.
• Define behavior at exactly cap (Nth allowed, N+1 rejected).
• Add tests filling to the cap boundary.

Acceptance criteria

• Count can never exceed max_cap.
• Exact-cap registrant succeeds; next reverts with typed error.
• Boundary tests added.

[gloskull]

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