feat(ffi): Add webui_protocol_tokens FFI function to extract CSS token names

[akroshg]

Adds webui_protocol_tokens — a new C-compatible FFI function that deserializes a WebUI protocol buffer and returns the CSS token name list as a
newline-delimited, heap-allocated string.

Motivation

Host languages (C#, Go, Python, C++) need to extract CSS design-token names from a compiled WebUI protocol without reaching into Rust internals. This
provides a safe, zero-dependency FFI entry point for that use case.

API contract

┌───────────────────────────────┬───────────────────────────────┬─────────────────────────────────┐
│ Input                         │ Return value                  │ webui_last_error                │
├───────────────────────────────┼───────────────────────────────┼─────────────────────────────────┤
│ Valid protocol with tokens    │ "token1\ntoken2\n…"           │ cleared                         │
├───────────────────────────────┼───────────────────────────────┼─────────────────────────────────┤
│ Valid protocol with no tokens │ "" (empty string, non-NULL)   │ cleared                         │
├───────────────────────────────┼───────────────────────────────┼─────────────────────────────────┤
│ Zero-length buffer (non-null) │ "" (empty protocol)           │ cleared                         │
├───────────────────────────────┼───────────────────────────────┼─────────────────────────────────┤
│ NULL pointer                  │ NULL                          │ "protocol_data is null"         │
├───────────────────────────────┼───────────────────────────────┼─────────────────────────────────┤
│ Invalid protobuf bytes        │ NULL                          │ "failed to parse protobuf: …"   │
└───────────────────────────────┴───────────────────────────────┴─────────────────────────────────┘

Caller must free the returned string with webui_free.

Changes

• crates/webui-ffi/src/lib.rs — New webui_protocol_tokens function following existing FFI patterns (catch_unwind, null-check, thread-local error,
  CString ownership transfer).
• crates/webui-ffi/include/webui_ffi.h — C header declaration with full # Safety documentation.
• crates/webui-ffi/tests/ffi_test.rs — 7 integration tests:
• Empty token list → ""
• Single token
• Multiple tokens (newline-delimited)
• Order and duplicates preserved
• Null pointer → NULL + error
• Zero-length buffer → "" (valid empty protocol)
• Invalid protobuf → NULL + error

[mohamedmansour]

Don't we need to support the webui-node and webui-wasm as well to expose this? For it to be complete.

[akroshg]

Fixed with latest update
Added protocol_tokens to both bindings in the latest push:

• webui-node: protocol_tokens(Buffer) → string[] via napi
• webui-wasm: protocol_tokens(string) → string[] via wasm_bindgen