Use unsafe Rust for catalyst

[yanganto]

Add unsafe feature for zero-memory-move catalyst operations

This PR introduces an optional unsafe feature that uses ManuallyDrop + ptr::read / MaybeUninit + ptr::write to avoid memory moves in the generated catalyst and substrate code.

Changes

derive/src/catalyst.rs

• Complex::decouple — gated into two implementations:
  • unsafe: wraps self in ManuallyDrop, reads fields via ptr::read to construct the catalyst and substrate without moving
  • safe: original destructuring approach
• Catalyst::bind — gated into two implementations:
  • unsafe: wraps self in ManuallyDrop, reads catalyst fields via ptr::read (substrate still uses __substrate_unpack() since those fields are private cross-crate)
  • safe: original destructuring approach
• to_ptr_read_stream / to_ptr_read_arg_stream parameterized with a source Ident

derive/src/substrate.rs

• __substrate_new — gated into two implementations:
  • unsafe: uses MaybeUninit + ptr::write + addr_of_mut to construct the struct field-by-field
  • safe: original struct literal construction
• __substrate_unpack — gated into two implementations:
  • unsafe: uses ManuallyDrop + ptr::read to extract fields without running Drop
  • safe: original tuple destructuring

Feature forwarding

• derive/Cargo.toml: added unsafe = [] feature
• lib/Cargo.toml: unsafe forwards to struct-patch-derive/unsafe
• complex-example/catalyst/Cargo.toml: unsafe forwards to struct-patch/unsafe, enabling cargo test -p catalyst --features unsafe for easy testing

Other

• flake.nix: check-catalyst now also runs cargo test -p catalyst --features unsafe
• README.md: documented the unsafe feature; replaced the "future" note with the actual implementation
• Bumped version to 0.12.7

Testing

All 9 tests pass both with and without the unsafe feature:

• cargo test -p substrate (3 tests)
• cargo test -p catalyst (3 tests, safe path)
• cargo test -p catalyst --features unsafe (3 tests, unsafe path)