Forbid unsafe across all crates and document 100% safe Rust

[powersemmi]

Description

Add #![forbid(unsafe_code)] to the proc-macro crate (ruststream-macros) and the CLI binary crate root (src/bin/ruststream/main.rs), so every crate in the workspace forbids unsafe, matching the core crate. The guarantee is compiler-enforced through the existing clippy and check steps (which build --workspace --all-targets --all-features and --no-default-features), so no separate CI gate is added: a stray unsafe block fails the build.

State the guarantee in the README with a "100% safe Rust" badge and a one-line note, so the property is visible to users.

No unsafe exists in the source today, so this is enforcement and documentation, not a refactor. Additive; no API or behavior change. Follows sqlx's stance of enforcing #![forbid(unsafe_code)] across all crates.

Fixes #128

Type of change

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (removing or renaming public API, changing a signature, tightening bounds, or
  raising the MSRV - a minor version bump pre-1.0)
• This change requires a documentation update

Checklist

• My code follows the project's style guidelines (just check passes: rustfmt, clippy, and
  cargo check with all features and with --no-default-features)
• I have performed a self-review of my own code
• I have made the necessary changes to the documentation (rustdoc and/or the docs site)
• My changes generate no new warnings (clippy runs with -D warnings)
• I have added tests that validate my fix or new feature
• New and existing tests pass locally (just test)
• Public items carry a compiling # Examples doctest, and user-facing changes are reflected in
  examples/ where applicable