ci: open source, align MSRV, and consolidate consumer checks

[forhappy]

Summary

• Make crabbuild/silo public and remove private/closed-distribution assumptions from repository, contribution, security, changelog, and release documentation.
• Make Rust 1.94.1 the single workspace MSRV inherited by silo-s3-core and silo-s3-client.
• Consolidate the external consumer compile checks under consumer-checks/ as one standalone Cargo workspace with one independent lockfile, while keeping it outside the root workspace.
• Complete the public CI/release workflows with package validation, strict checks, concurrency cancellation, least-privilege checkout, unified MSRV checks, RustFS cleanup, and public GitHub release bundles.

Validation

• cargo fmt --manifest-path Cargo.toml --all -- --check
• cargo fmt --manifest-path consumer-checks/Cargo.toml --all -- --check
• cargo check --locked --manifest-path consumer-checks/Cargo.toml --workspace --all-features
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• cargo test --locked --workspace --all-features
• cargo package --locked --workspace --all-features
• python3 spec/prolly-s3/conformance/verify.py
• scripts/check_dependency_security.sh
• scripts/check_clean_downstream.sh
• Live RustFS qualification: 14 passed, 8 intentionally ignored
• Workflow YAML parse validation

The PR remains draft until the public GitHub Actions run completes successfully.