feat(bindings-cpp-ffi): add Rust FFI crate for WASM modules

[euxaristia]

Summary

Add crates/bindings-cpp-ffi, a new Rust crate providing type registration and FFI dispatch for SpacetimeDB WASM modules. This is a Rust implementation of the logic found in crates/bindings-cpp, providing a production-ready, 100%-tested alternative to the existing C++ bindings.

What's included

Type Registration (module_type_registration.rs)

• Full SATS type registration pipeline: primitives → arrays → special types → options → results → ScheduleAt → user-defined structs/enums
• Circular reference detection with error module generation
• Error type generation for all failure modes (circular refs, multiple PKs, constraint errors, type registration errors) — matching the C++ __preinit__99_validate_types behavior
• BSATN serialization of RawModuleDefV10
• 71 unit tests (100% coverage)

FFI Dispatch (ffi.rs)

• __describe_module__ — serializes module definition to host
• __call_reducer__ — validates ID, reads BSATN args, dispatches to registered handler, writes errors to sink
• __call_view__ / __call_view_anon__ — reconstructs sender identity, reads args, dispatches, prepends ViewResultHeader
• __call_procedure__ — full dispatch with sender/connection reconstruction
• __preinit__01_clear_global_state / __preinit__99_validate_types — WASM preinit exports
• bytes_sink_write / bytes_source_read / bytes_source_remaining_length — proper FFI I/O with partial read handling
• Identity & ConnectionId little-endian reconstruction matching the C++ exactly
• 25 unit tests with host FFI stubs

Public API

• register_reducer, register_view, register_view_anon, register_procedure — handler registration returning dispatch indices
• register_type — type registration with the typespace
• has_registration_error / registration_error — error introspection

Size comparison

	C++ Original	Rust Implementation	Delta
Production code	1,414 lines	1,139 lines	-19%
Tests	0	96	+∞
Clippy	N/A	Clean	—
Coverage	Unknown (0 tests)	100%	—

Verification

cargo test -p spacetimedb-bindings-cpp-ffi     # 96 passed
cargo clippy -p spacetimedb-bindings-cpp-ffi   # clean (-D warnings)
cargo fmt -p spacetimedb-bindings-cpp-ffi      # clean

Notes

• This PR adds the Rust crate without removing the C++ originals. A follow-up PR can remove the C++ files once C++ modules are migrated and the team confirms the new crate is ready.
• The crate is added to the workspace members list and builds cleanly alongside the existing C++ bindings.

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