fix(signing): publish round state before messages

[piotr-roslaniec]

Problem

The final full-suite race run found TestRounds3to5CTWiring reading round-5 state concurrently with round5.Start. round5.Start emitted its outbound message before storing si, rx, and the rest of its completed local state, so receiving the message did not synchronize those writes. The same publication-order variant existed for DTelda in round 7.

Fix

Store all round-local state before publishing the corresponding outbound message in rounds 5 and 7. Wire bytes and arithmetic are unchanged.

Evidence

• go test -race ./ecdsa/signing -run ^TestRounds3to5CTWiring$ -count=3
• go test -race -shuffle=on -count=1 ./ecdsa/signing

[piotr-roslaniec]

Final evidence at f73a2c135e0b1b71a466f07742cac58fc0d0f2f2:

• Root cause: rounds 5 and 7 published outbound messages before their final local-state writes.
• Targeted round-5 race regression passed 3 consecutive runs.
• Full signing package race+shuffle passed.
• Final repository-wide race+shuffle suite passed, including keygen (2058s) and signing (397s).
• Go-fmt run 36697246316 and full Go Test run 36697246297 passed.