turbine and leader: accelerate ingestion and block production

[7layermagik]

Accelerate shred production and ingestion, prepare transactions during arrival, and reduce leader packing overhead.

• FEC encoding/recovery acceleration with signed-Merkle authentication of reconstructed shreds.
• Early signature verification and message preparation, reduced retention scans, and completion journaling with failure propagation.
• Prepared leader transactions, admission prechecks and near-limit synthetic block tests, preserving priority and cost limits.

Combines the former #287 and #290 scopes into one review against alpenglow-dev. Replaces #259 and the production portion of #279. Commits and authorship are preserved. Replay-side incremental execution and its repair integration are in #292.

Validation: full turbine, sigverify and blockprod suites passed on this exact source tree; repair-sim built. No new performance measurement or deployment was performed during consolidation. Historical component benchmarks are not claims about live voting latency.