feat: upgrade Summit checkpoint decoding and dependency stack

[matthias-wright]

Changes:

• Pinned Summit to b1651e7eaef0378815359ffdc4631316f78f03c4.
• Aligned Commonware to 2026.9.0; verified all 19 resolved Commonware packages match.
• Added Rust 1.95.0 toolchain pin and updated CI to use locked builds, fixture regeneration checks, and a release build.
• Added a deterministic new-format fixture with:
  • Current validator cap 256, queued cap 512.
  • A verified BLS certificate.
  • Distinct prior-capture, checkpoint, and terminal execution identities.
• Added 12 regression tests covering production decoding, corrupted/mismatched artifacts, Reth identity checks, bundle writing, and exact manifest/archive hashing.
• Added an isolated previous-decoder regression harness.