chore: bump SP1 to 6.8.1

[fakedev9999]

Summary

Update the SP1 host, guest, Docker, and CI pins from 6.8.0 to 6.8.1.
Refresh both lockfiles.
Reth remains at 2.2.0.

Motivation

SP1 6.8.1 includes an ELF loader fix.
Downstream users need an RSP release with matching SP1 packages.

Validation

• cargo tree --locked passed for the workspace and guest.
• cargo fmt --all -- --check passed.
• cargo build --locked --all --all-targets generated the guest ELF and passed.
• cargo clippy --locked --all --all-targets -- -D warnings passed.
• Focused library tests passed for the executor, primitives, MPT, and provider packages.

RPC-backed integration tests remain for GitHub CI.
This validation did not generate a cryptographic proof.

[github-actions]

Block 20600000	Base Branch	Current PR	Diff	Diff (%)
Total Cycle Count	282,127,664	282,127,445	-219	-0.00
Inputs deserialization	72,429,324	72,432,794	3,470	0.00
Initialize Witness DB	61,250,381	61,249,545	-836	-0.00
Recover Senders	7,944,806	7,944,806	0	0.00
Header Validation	1,404,771	1,404,777	6	0.00
Block Execution	84,329,563	84,330,999	1,436	0.00
Block Validation	3,983,009	3,983,009	0	0.00
State Root Computation	32,845,426	32,845,423	-3	-0.00
Syscall Count	208,720	208,720	0	0.00
Prover Gas	350,706,690	350,705,531	-1,159	-0.00