feat(ffi): add provekit-ffi

[Bisht13]

Summary

Adds a new provekit-ffi crate that provides C-compatible FFI bindings for ProveKit, enabling integration with multiple programming languages and platforms including mobile (iOS, Android), desktop, web, and embedded systems.

Features

• C ABI Compatibility: All functions use C-compatible types and calling conventions
• Memory Management: Safe buffer management with explicit allocation/deallocation via PKBuf
• Multiple Output Formats: Support for binary file output and JSON in-memory output
• Error Handling: Comprehensive error codes (PKError enum) with clear semantics
• Cross-Platform: Compiles as a static library for iOS, Android, desktop, and embedded targets

API

Functions

Function	Description
pk_init()	Initialize the library (call once before using other functions)
pk_prove_to_file()	Generate proof and write to file
pk_prove_to_json()	Generate proof and return as JSON string in memory buffer
pk_free_buf()	Free buffers returned by ProveKit functions

Error Codes

Code	Value	Description
PK_SUCCESS	0	Operation successful
PK_INVALID_INPUT	1	Invalid input parameters
PK_SCHEME_READ_ERROR	2	Failed to read scheme file
PK_WITNESS_READ_ERROR	3	Failed to read witness/input file
PK_PROOF_ERROR	4	Failed to generate proof
PK_SERIALIZATION_ERROR	5	Failed to serialize output
PK_UTF8_ERROR	6	UTF-8 conversion error
PK_FILE_WRITE_ERROR	7	File write error

Additional Changes

• Updated CI workflow to use nightly-2025-10-20 matching rust-toolchain.toml
• Fixed minor clippy warnings in provekit-common

[yash25198]

Returns 'static lifetime but the underlying C string has no such guarantee if the caller frees the string while Rust code holds the reference.

[yash25198]

If the original Vec had capacity > len, we're reconstructing with wrong capacity.
example :

let mut v: Vec<u8> = Vec::with_capacity(100);  // capacity = 100
v.push(1);
v.push(2);
v.push(3);
// Now: len = 3, capacity = 100

and then in types.rs

pub fn from_vec(mut v: Vec<u8>) -> Self {
    let ptr = v.as_mut_ptr();
    let len = v.len();          // len = 3
    std::mem::forget(v);        // capacity (100) 
    Self { ptr, len }           // PKBuf { ptr, len: 3 } 
}

and then pk_free_buf reconstructs with wrong capacity

drop(Vec::from_raw_parts(buf.ptr, buf.len, buf.len));
// len=3    cap=3 (was 100)

the allocator allocated 100 bytes but only 3 bytes were freed

[yash25198]

unreachable condition

[yash25198]

unused dependency