feat(sdk): add transport-free CXX bindings for C++ embedders

[PastaPastaPasta]

Issue being fixed or feature implemented

Third and last PR of the transport-free-embedder series; stacked on #4619 (review the last commit, feat(sdk): add transport-free CXX bindings for C++ embedders). Supersedes #4416. Draft until #4619 merges.

Dash Core's platform GUI (dashpay/dash#7512, #7623 onward) currently carries a private Rust/CXX bridge and pins individual Platform crates. This crate gives Platform ownership of that embedding surface: proof verification, DPP decoding and state-transition construction for an application that owns its own DAPI transport, quorum-key sync and private keys.

The earlier #4416 was withdrawn after review found (a) a malicious getDocuments response could abort the embedding process before the quorum signature was checked (fixed upstream in #4618), (b) cxx turns a Rust panic into a deterministic process abort and nothing caught them, (c) a second lockfile under standalone/ that had already drifted from the workspace pin so the documented --locked build failed, and (d) ~600 lines of test vectors duplicating the proof-verifier corpus.

What was done?

packages/rs-platform-cxx (dash-platform-cxx), a thin cxx bridge over the workspace's own crates: verification is drive-proof-verifier's FromProof impls including #4619's FromProof<GetDocumentsRequest>; document assembly is dash-platform-queries' builders; signing is dpp's Signer trait implemented over a C++ digest callback (WalletSigner) so private keys never cross the FFI; quorum keys and system contracts come from a ContextProvider the embedder feeds from its synced LLMQ state.

Trust boundary (every byte is untrusted DAPI output, and grovedb replay necessarily precedes the signature check):

• every extern "Rust" entry point runs under catch_unwind and reports a panic as rust::Error; build.rs refuses panic = "abort" builds, where that guard would be compiled out;
• requests, responses and decoder inputs are capped at verify::MAX_MESSAGE_BYTES before decoding; asset-lock transaction/islock bytes at 2 MiB;
• set_context(network, platform_quorum_type, protocol_version, activation_height) records the network's Platform LLMQ type; a proof naming any other quorum type is refused before its key is looked up, so keys the embedder pushes for other purposes can never sign Platform state. update_quorum_keys replaces the whole set;
• a response claiming a protocol version this build does not know is refused rather than verified under a guessed one; transitions are built and stored documents decoded under the network's protocol version from set_context, not PlatformVersion::latest();
• the signer refuses non-ECDSA key types (the callback can only produce compact secp256k1 signatures); built transitions pass the same ensure_valid_state_transition_structure dash-sdk runs before broadcast.

Packaging follows the workspace's FFI crates: an ordinary member with no nested lockfile (embedders cargo vendor from the root and cargo build -p dash-platform-cxx), build.rs stages include/dash/platform/{ffi.h,signer.h} and include/rust/cxx.h under target/<profile>/ the way rs-sdk-ffi stages its cbindgen header, cxx = "1.0" unpinned. cxx rather than cbindgen because the surface is dominated by nested byte vectors and fallible calls; the README states that trade-off.

How Has This Been Tested?

• 38 Rust tests: FromProof end-to-end positives over the shared proof-vector corpus (../rs-drive-proof-verifier/tests/vectors: identity nonce, contract nonce, contested active/finished/absent) plus negatives for tampered signature, unknown quorum hash, non-Platform quorum type, tampered block-id hash, tampered grovedb proof, each signed metadata field individually, unknown protocol version, request/proof identity mismatch, wrong-shape proof, oversized and garbage input; byte-exact state-transition builds against the rs-dpp-generated vectors plus non-ECDSA key, wrong signature size, invalid DPNS label refused before signing, signer refusal; decoders incl. oversized input.
• scripts/cxx-smoke.sh (in CI) compiles tests/cxx_smoke.cc against the staged headers and archive and runs it: context setup, garbage input throws rust::Error rather than aborting, and all five st_build_* builders driven through the real bridge with a declining WalletSigner.
• cargo clippy --all-targets --all-features -- -D warnings and cargo machete: clean. The transport-free CI cut asserts hyper/rustls/tower stay out of the crate's tree.

Breaking Changes

None for the workspace (new crate). For the Dash Core consumer of the withdrawn #4416: set_context gained two parameters and update_quorum_keys lost its per-call type; standalone/ and install.sh are gone (install from the staged include/ and libdash_platform_cxx.a).

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

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[PastaPastaPasta]

Disposition (not closing yet): superseded by #4633, the same crate rebuilt as a bridge over dash-sdk (networking included). The context provider, signer callback, state-transition builders and the byte-exact test vectors carry over; the verify-from-bytes layer is replaced by the SDK's Fetch/FetchMany, and the crate adds the chain-id binding and ChainLock freshness floor the SDK lacked. Live-testnet evidence and measurements in dashpay/dash#7512.

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🤖 Posted autonomously by Claude on behalf of pasta.