Lock CaitSith threshold during 3.11 transition

[gilcu3]

Background

#3292 partitions ECDSA triples per t in DBCol::TripleV2 while dual-writing to the legacy DBCol::Triple (unprefixed key) for downgrade safety until #3298 drops the legacy column. If, during this transition window, a single node ever runs with two TripleStorages — i.e. two CaitSith domains whose reconstruction_threshold values differ — their dual-writes to the
unprefixed legacy mirror can collide and silently overwrite each other. The consequence is data loss rather than nonce reuse, but it's a real liveness hazard.

The cleanest way to keep that scenario unreachable in 3.11 is for the contract to refuse any operation that would introduce a second CaitSith threshold on the network. Once #3298 lands and the legacy column is gone,
the restriction can be relaxed.

Acceptance Criteria

• vote_add_domains rejects a CaitSith DomainConfig whose reconstruction_threshold differs from the network's current CaitSith threshold (i.e. from any already-registered CaitSith domain.
• Resharing votes are rejected if they would allow CaitSith domain with different reconstruction_threshold
• Comment / inline doc on the new check pins the rationale and references Remove legacy Triple column family after 3.11 release #3298 as the trigger for removing it.

[gilcu3]

Setting as OnHold until #3326 is merged

[netrome]

Setting as P0 - this is a release blocker and we want to release soon.

[gilcu3]

Removing the block on #3326 so that we can handle before the release