feat(sdk): add transport-free CXX bindings

[PastaPastaPasta]

Issue being fixed or feature implemented

Dash Core's Platform GUI currently carries a private copy of its Rust/CXX bridge and pins individual Platform crates. This makes the embedding ABI, proof-verification logic, decoders, state-transition builders, and dependency closure owned by the consumer instead of Platform.

This stacked PR gives Platform ownership of that transport-free C++ embedding surface. It is based on #4433 (which stacks on #4389) and assumes #4389 and #4433 merge before this PR is retargeted to v4.2-dev. (#4388 and #4434 have already merged.)

What was done?

• Added the dash-platform-cxx workspace package with the existing platform_ffi ABI for proof verification, DPP decoding, state-transition construction, quorum context, and callback-based wallet signing.
• Added 31 Rust tests and the proof/decoder/signing vectors previously maintained with the Dash Core integration.
• Added a standalone manifest and lockfile so downstream build systems can vendor only this package's dependency closure.
• Added an install contract for the static archive and public/generated CXX headers.
• Added a real C++ link-and-run smoke test and CI checks that reject native DAPI transport dependencies.
• Integrated the package with workspace metadata, package filters, Docker package discovery, feature checks, and nightly CI.

After this lands, PastaPastaPasta/dash#67 will consume the installed archive and headers through Dash Core's depends system and remove its private Rust/CXX implementation.

How Has This Been Tested?

• cargo test -p dash-platform-cxx --locked (31 tests)
• cargo clippy -p dash-platform-cxx --all-targets --locked -- -D warnings
• cargo check --manifest-path packages/rs-platform-cxx/standalone/Cargo.toml --locked
• standalone debug C++ link-and-run smoke test
• standalone release build and install-layout verification
• workspace and standalone dependency-tree checks for rs-dapi-client, Hyper, Rustls, Tower, and Reqwest
• cargo machete
• cargo fmt --all -- --check
• shellcheck packages/rs-platform-cxx/install.sh packages/rs-platform-cxx/test-cxx-link.sh
• YAML parsing and git diff --check

Breaking Changes

None. The existing platform_ffi namespace and bridge ABI from the downstream integration are preserved.

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

This pull request was created by Codex.

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