Remove unused derive and core APIs

[gold-silver-copper]

Summary

• Remove the unused public rig_derive::ProviderClient derive macro and its deluxe dependency chain.
• Remove unused Extractor::{get_inner, into_inner} and the always-failing TryFrom<String> for Nothing.
• Replace the stale root-facade docs version with a valid non-pinned Cargo requirement.
• Add breaking-removal notes at root, rig-core, and rig-derive changelog scopes.

Closes #2074.
Closes #2075.

Validation

• cargo fmt
• cargo fmt --check
• cargo check -p rig-derive -p rig-core --all-features
• cargo check --workspace --all-features
• cargo check -p rig-core --features derive --locked
• cargo check -p rig-core --features wasm --locked
• cargo test -p rig-derive --all-features
• cargo test -p rig-derive --locked
• cargo test -p rig-core --lib --locked
• cargo metadata --locked --no-deps
• exact docs-snippet cargo metadata --no-deps validation
• cargo tree -i deluxe confirms no remaining dependency
• git diff --check

Not run

• Full workspace test suite
• Full workspace clippy
• Docs build
• Live provider/network tests