hexeract: facade crate re-exports

[pierrick-fonquerne]

Wire the hexeract crate as a single entry point so users can add one dependency and access every public API.

Scope

• Re-export all public items from hexeract-core, hexeract-mediator and hexeract-macros.
• Feature flags aligned with hexeract-core (serde).
• Documentation landing page with a minimal end-to-end usage example.

Acceptance criteria

• use hexeract::{Command, Mediator, handler} compiles without additional dependencies.
• The serde feature on hexeract activates the matching flag on hexeract-core.
• cargo doc -p hexeract --no-deps generates documentation with the usage example.
• cargo clippy -p hexeract -- -D warnings passes.