Remove protocols feature from core crate

[KaiCode2]

Summary

• remove the old protocols feature and in-crate AMM/protocol modules, tests, fixtures, examples, and storage-key benchmark
• make immutable metadata token-decimals-only with cache format version 2
• rename remaining storage purge helpers to contract terminology and update docs, CI, examples, and benchmarks around the generic crate boundary
• add a release-surface guard test to prevent the removed protocol surface from returning

Validation

• cargo test --test public_release_surface
• cargo test --all-targets
• cargo fmt --all --check
• cargo clippy --all-targets --no-deps -- -D warnings
• cargo test --doc
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps
• cargo bench --no-run
• cargo package --locked --allow-dirty
• cargo +1.88.0 check --lib --locked

A separate review-agent pass found no core correctness blockers. Its release-facing benchmark/wording findings were addressed before opening this PR.