Add JSON string dump C binding and avoid function lookup panic

[samyycX]

What changed

• Added S2BinLib::dump_strings_to_json to rescan a loaded binary and write printable ASCII strings as a deterministic pretty-printed JSON object mapping string values to RVAs.
• Added the global C API s2binlib_dump_strings_to_json(binary_name, output_path), including UTF-8/null validation, automatic binary loading, error-code propagation, the static header declaration, and a README example.
• Replaced the unchecked vtable lookup in find_func_start_rva with safe optional handling. It now falls back to the xref-derived function start and returns an error when neither source has a candidate.
• Added focused tests for JSON output ordering/RVAs, xref fallback, and the no-candidate error path.

Why

The existing string dumper only populated an in-memory cache, so C consumers could not persist the result. The function-start lookup also unconditionally unwrapped an optional vtable candidate, which could panic when a function was discoverable only through xrefs or when no candidate existed.

Impact

C callers can now write all discovered printable ASCII strings for a binary to a caller-selected JSON path with:

s2binlib_dump_strings_to_json("server", "server_strings.json");

Function-start lookup now fails through the existing Result/C error-code path instead of panicking.

Validation

• cargo test -p s2binlib s2binlib::tests:: — 3 passed
• cargo check --workspace — passed (existing warnings remain)
• cargo build -p s2binlib_binding — passed
• Verified s2binlib_dump_strings_to_json is exported from the generated Windows DLL with dumpbin /exports

The repository's full pre-existing unit suite still has three unrelated failures: test_find_vtable depends on F:/cs2server/..., and two Windows JIT tests fail while restoring memory protection.