refactor!: remove obsolete crate features

[mbrobbel]

Summary

• remove the non-functional derive feature and narrow-derive dependency from narrow
• remove empty default feature declarations from all workspace crates
• make the existing README-based crate documentation unconditional
• collapse CI onto one feature-independent build, test, Miri, and coverage configuration

Breaking change

narrow no longer defines a derive Cargo feature. The feature compiled an empty proc-macro crate but did not re-export or enable any API.

The narrow-derive and narrow-ffi packages remain workspace members. A future derive feature can be introduced when it has a concrete macro re-export; FFI remains a separate integration crate to avoid a cyclic dependency.

CI impact

The Miri job previously ran the same test suite twice for 33 seconds per feature mode. It now runs once. The redundant MSRV feature-mode check is also removed.

Validation

• Cargo metadata reports empty feature maps for all three packages and no dependencies for narrow
• cargo check --workspace --all-targets
• cargo test --workspace --all-targets (98 tests)
• cargo test --workspace --doc (100 doctests)
• cargo clippy --workspace --all-targets -- -Dwarnings
• cargo fmt --all -- --check
• cargo llvm-cov --workspace --all-targets --codecov (98 tests and report generation)