fix(network,sync): eliminate OMP data races on shared working variables

[adequatelimited]

Summary

• Split the shared int result in scan_quorum() into two thread-private variables (weight_cmp, contrib) — eliminates a real data race
• While in the neighborhood, moved peer and ecode in catchup() to the same function-top + private() clause style, matching the codebase convention

Problem

scan_quorum() used a single shared result variable for two semantically different purposes — weight comparison inside OMP_CRITICAL_() and peer-contribution counting outside any critical section. Thread A writing result = cmp256(...) inside the critical section while Thread B wrote result = 0 / result++ outside produced a genuine C11 data race.

Empirical evidence

Ran a TSan-instrumented build against a live mainnet scan:

• Before fix: 71+ race reports in the scan phase, including multiple on result
• After fix: 61 reports, zero on result / weight_cmp / contrib
• The 10 missing reports correspond exactly to the F-30 access pattern

Remaining reports are separate issues (scanidx/netplistidx/Rplistidx shared-state, peer.c shuffle32/addpeer, resync() at sync.c:212) — out of scope for this PR, could be new audit findings.

Changes

src/network.c — scan_quorum()

• Removed shared int result; at function top
• Added two thread-private variables: int weight_cmp; int contrib;
• Added them to the OMP private() clause
• Replaced all uses of result with the appropriate per-purpose variable

src/sync.c — catchup()

• Moved word32 peer; and int ecode; from inside-the-OMP-region declarations to function top
• Added both to the OMP private() clause
• Assignments (peer = plist[OMP_THREADNUM], ecode = VEOK) now happen at the top of the parallel region body since OMP private() doesn't initialize
• No behavior change; purely style consistency

Test plan

• Verify make (mochimo binary) passes
• Run TSan-instrumented sync: no races flagged on weight_cmp, contrib, peer (catchup), or ecode (catchup)
• Verify scan_quorum produces same quorum membership as before under non-adversarial conditions

Closes #110