[fakedev9999]

What

• Subscribe execute-only execution to SP1's bounded guest stderr channel.
• Include the latest non-empty stderr snapshot in existing failure metadata for non-zero exits.
• Assert the panic message and exit code in the existing execute-only E2E scenario.
• Update the SP1 dependency set to the published v6.6.0 crates.

Why

Non-zero guest exits include the typed failure cause and exit code, but omit the guest panic message and location.

Validation

• cargo fmt --manifest-path Cargo.toml --all -- --check -v
• cargo clippy --manifest-path Cargo.toml --release --all-targets -- -D warnings -A incomplete-features
• cargo test --manifest-path Cargo.toml --workspace --release
• cargo check --workspace --release
• cargo sort -g -w --check
• SP1_CLUSTER_CPU_ONLY=1 RUST_LOG=info cargo run --release --bin sp1-test-cluster -- run execute-only