Adds the `core` feature for wasm32 / CF-Workers consumers. All async-IO
deps now optional behind `tokio-runtime`. `default` preserves the
alpha.2 surface bit-for-bit.

cargo check (default): PASS
cargo check --no-default-features --features core: PASS
cargo test --lib: 236 tests PASS
cargo publish --dry-run -p solid-pod-rs: PASS

Refs: ADR-076, ADR-078, PRD-004.