Document Rust applications and publish versioned SDK reference

[Amaury Chamayou (achamayou)]

Motivation

Stacked on #8200, targeting achamayou-rust-interface-exploration.

Rust application authors need a runnable path from CCF setup to authenticated requests, and API contracts comparable in quality to the existing C++ documentation. The initial Rust guide describes the interface but does not provide a source-backed walkthrough or generated API reference.

Implementation summary

• Expand the existing guide with sample-backed Cargo/CMake and handler snippets, source-tree and installed-SDK instructions, sandbox requests, expected responses, and explicit experimental/capability limits.
• Document the safe SDK's ownership, authentication, transaction, error and serialization contracts. Add executable and compile-fail doctests, deny missing documentation and broken intra-doc links, and hide ABI implementation details without removing exported symbols.
• Generate native rustdoc alongside each Sphinx documentation version, following the existing TypeDoc model. Build only the dependency-free application SDK, copy its complete output, and validate generated page/anchor links, including cached doctrees. Older versions without the SDK skip generation; SDK generation failures are fatal.

Local validation passed: the full warnings-as-errors Sphinx build, 10 documentation-helper tests, SDK unit tests and doctests on Rust 1.90.0 and 1.93.1, sample cargo check, and scripts/ci-checks.sh. The native Debug basic_rust target also builds, and the ccf_app_rust_test and e2e_basic_rust CTest selections pass. The helper tests are wired into the existing documentation CI step; SDK doctests use the existing Rust test registration.

Safety and compatibility

No application or bridge runtime behavior, ABI, ledger encoding, consensus, recovery, or mixed-version behavior changes. Rust edits add documentation, doctests, documentation lints and documentation-only visibility attributes; sample edits add snippet markers.

Rust documentation generation does not build a CCF executable. SKIP_RUSTDOC is an explicit local-preview escape hatch, not used in publishing or CI. Native rustdoc retains its own presentation and search; Sphinx remains the versioned entry point and conceptual guide.