Nimbus v26.7.0 Cuts CPU, Memory, and Fixes Stability Leaks

Nimbus consensus client v26.7.0, a medium-urgency release, optimizes CPU and memory usage. Adds backfilled column reconstruction, correct block request errors, and mempool column reassembly. Fixes libp2p peer leakage and unbounded column quarantine growth. Update sooner rather than later for stability fixes.