chore: move the test fixtures into testenv and expose the contract binaries as an API

[bennyhodl]

Summary

Fold testconfig/ into the ddk-testenv crate and expose the checked-in contract binaries as an API, so a test names a contract state instead of spelling out a ../../../testconfig/... path. Also deletes the plans/ folder.

Changes

• Move contract_binaries/, nostr-relay.toml, sample_contract.json, and scripts/ from testconfig/ to testenv/; update docker-compose.yaml, the CI workflow, .gitignore, the generator script, and the docs to match.
• Add testenv/src/contract_binaries.rs: a ContractBinary enum (Offered … Closed, with ALL and name()), contract_binary(state) and legacy_contract_binary() that return the bytes via include_bytes!, and contract_binaries_dir() for the generator. Not feature-gated; it has no dependencies.
• Rewrite every include_bytes! and CARGO_MANIFEST_DIR fixture loader in ddk (ser, sled, wallet command, postgres store and row, balance and enumeration tests) to use the new API. The FIXTURES const in ser.rs now iterates ContractBinary::ALL.
• Remove the five fixtures under old/ that nothing loaded and that stopped deserializing long ago. Keep the one that still matters, an offered contract from before contract_flags, as contract_binaries/legacy/Offered; the three tests that assert it still reads are renamed to say legacy.
• Drop two blocks of commented-out code in the sled tests that pointed at files that do not exist.
• Delete plans/custom-tlv-stream.md.

Testing

• cargo test -p ddk --features sled,postgres --lib for the ser, wallet, sled, and postgres modules: all pass.
• cargo test -p ddk --tests --no-run: the integration tests compile.
• cargo test -p ddk-testenv --doc: passes.
• cargo +nightly fmt --all applied.