Add active session snapshot restoration protocol

[kevinyang372]

Summary

• add the versioned MonolithicV1 active-session restore snapshot envelope and immutable storage descriptor
• negotiate snapshot schema, reducer, storage layout, and compression capabilities
• add prepared publication receipts, publication acknowledgements, resume cursors/floors, and classified resync reasons
• preserve legacy wire payload compatibility with golden fixtures

Performance context

A downstream local Redis benchmark using valid protocol events and fixed restore watermark H measured the expected suffix-scaling behavior:

• 50% snapshot position: approximately 2x faster server Redis-read plus serialization path
• 90% snapshot position: approximately 9-10x faster
• 99% snapshot position: approximately 37-89x faster depending on event profile
• measured transfer-byte reduction ranged from 32% to 97% across 1K/10K PTY-heavy and mixed-agent fixtures

These figures exclude GCS authorization/download and client decode/hydration; the downstream server and client PRs contain the benchmark harnesses for those layers.

Rollout safety

This PR only defines the wire contract. Snapshot publication, restore, storage, and Redis compaction remain disabled in downstream repositories. Production shadow equivalence and retained-session sampling remain rollout blockers.

Validation

• RUSTUP_TOOLCHAIN=1.88.0 cargo test --all-targets (13 passed)
• RUSTUP_TOOLCHAIN=1.88.0 cargo clippy --all-targets --all-features -- -D warnings
• RUSTUP_TOOLCHAIN=1.88.0 cargo fmt --all -- --check
• git diff --check

Artifacts

• Implementation plan
• Development conversation