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. It re-implements the logic from
crates/bindings-cpp (specifically module_type_registration.cpp,
module_exports.cpp, and Module.cpp) in Rust.

Motivation: The existing C++ bindings have no internal unit tests — all
existing tests are integration-level (WASM compile → publish to server). This
makes it impossible to test type registration edge cases, error paths, and FFI
dispatch in isolation, and means iteration requires a full Emscripten +
SpacetimeDB server cycle. A Rust implementation gives us:

• Fast, offline, deterministic unit tests for the entire type registration and
  dispatch pipeline
• Shared types with spacetimedb-lib, spacetimedb-sats, and
  spacetimedb-primitives (no hand-rolled mirrors of Rust types in C++)
• A maintenance path fully within the Rust toolchain

Replacement intent: Yes. If the team is happy with this implementation, a
follow-up PR would remove crates/bindings-cpp and wire the SDK macros to this
crate instead. This PR only adds the crate — nothing is removed.

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% line 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 —
  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%
Unit tests	0	96	+∞
Integration tests	~60 modules (type isolation)	—
Clippy	N/A	Clean	—
Line coverage	Unknown	100%	—

Existing C++ test infrastructure

The C++ bindings do have integration-level tests:

• Type isolation test suite (tests/type-isolation-test/): 60 test
  modules that validate WASM compilation and publish to a running server
• Client comparison suite (tests/client-comparison/): validates that
  Rust and C++ bindings produce equivalent generated SDK clients and schemas
• Smoketest (crates/smoketests/): test_quickstart_cpp exercises the
  full quickstart flow with a C++ server module

However, there are no unit tests covering the type registration or FFI
dispatch internals. The CTest target (test_bsatn) in CMakeLists.txt
references source files that don't exist on disk.

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

• Added to the workspace members list; builds cleanly alongside existing
  C++ bindings
• No C++ code is removed in this PR — that would be a follow-up

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[euxaristia]

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[bfops]

Hi @euxaristia, what's the goal of this PR? Why is this version preferable to what's there? Is it intended as a replacement?

[euxaristia]

@bfops rewrites the C++ binding layer in Rust. The C++ side has integration tests (type-isolation, client-comparison) but zero unit tests on the internal logic, so edge cases/errors aren't covered. This adds 96 unit tests at 100% coverage, uses shared Rust types directly, and keeps everything in one toolchain.

Yes, intended as a replacement, but this PR only adds the crate — nothing removed. Follow-up would swap it in if the team's on board.