v3.1.0-beta -- audit remediation pre-release

Pre-release consolidating the F-series audit remediations merged into master since 3.0.3. See CHANGELOG.md for the detailed entry.

Summary

21 correctness fixes across consensus-adjacent and network-handling code paths:

• Determinism / consensus alignment: F-02 (block-declared fee), F-09 (deterministic quorum), F-14 (locale-free reference validation)
• Data races: F-10, F-30, F-32
• Remote-triggered vulnerabilities: F-13 (unknown TX types), F-18 (unbounded file receive), F-29 (oversized peer lists)
• Error handling: F-07 (balance overflow), F-21 (weight on failure), F-23 (read_tfile error disambiguation), F-31 (socket leak)
• Platform / ABI: F-11 (32-bit file offsets), F-15/F-16/F-27 (unaligned casts)
• Maintainability: F-06 / F-12 / F-20 (replace/guard system() calls), F-26 (remove dead read)

Also includes CI workflow runner update (ubuntu-20.04 → ubuntu-22.04).

No protocol or consensus rule changes; all items are behavioral corrections on existing code paths. Operators: test this release in a non-production environment before running against mainnet.

F-01, F-04 analyzed and closed as not-a-bug. F-08 reclassified as a feature request. See linked issues in CHANGELOG for details.