Implement UniFFI proc-macro FFI bindings for uts-core

[Copilot]

Completes the uts-ffi-binding crate with UniFFI proc-macro bindings, replacing the stub implementation. Exposes the core timestamp codec, attestation types, and opcode execution to foreign languages (Python, Kotlin, Swift).

FFI surface

Derived from analyzing the common API surface across the TS/Python/Go/Rust SDKs:

• Object — DetachedTimestamp: opaque handle wrapping VersionedProof<DetachedTimestamp> behind RwLock for thread-safe mutation
  • new(data) constructor (decode from OTS bytes)
  • encode(), header(), attestations(), is_finalized(), display(), purge_pending()
• Enum — Attestation (Bitcoin/Eas/EasTimestamped/Pending/Unknown), DigestOp (Sha1/Ripemd160/Sha256/Keccak256)
• Record — DigestHeader (kind + digest bytes)
• Error — UtsError as #[uniffi(flat_error)] with DecodeError/EncodeError/InvalidOperation
• Free functions — uts_execute_op, uts_validate_pending_uri, uts_digest_output_size, uts_opcode_name, uts_magic_bytes

Usage

Pure proc-macro flavor — no UDL files, no build.rs:

uniffi::setup_scaffolding!();

#[derive(uniffi::Object)]
pub struct DetachedTimestamp {
    inner: RwLock<VersionedProof<CoreDetachedTimestamp>>,
}

#[uniffi::export]
impl DetachedTimestamp {
    #[uniffi::constructor]
    pub fn new(data: Vec<u8>) -> Result<Arc<Self>, UtsError> { /* ... */ }
    pub fn attestations(&self) -> Result<Vec<Attestation>, UtsError> { /* ... */ }
}

#[uniffi::export]
pub fn uts_execute_op(op_tag: u8, input: Vec<u8>, immediate: Vec<u8>) -> Result<Vec<u8>, UtsError> { /* ... */ }

Changes

• crates/ffi-binding/Cargo.toml — added thiserror dep, sha2 dev-dep, "lib" crate-type for testing
• crates/ffi-binding/src/lib.rs — full rewrite with UniFFI proc-macro annotations, type conversions between core and FFI-safe types, 20 unit tests

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