fix(ffi): remove C header requirement from Zig build

- Remove mandatory installFile for include/proven.h
- Fix unused parameter error in proven_header_build_csp
- The Zig FFI exposes C ABI but doesn't require C headers
- Modern consumers (Zig, Rust via zig-cc) work without headers

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Jonathan D.A. Jewell

ffi/zig/build.zig

@@ -112,8 +112,11 @@ pub fn build(b: *std.Build) void {
     all_wasm_step.dependOn(wasm_browser_step);
     all_wasm_step.dependOn(wasm_wasi_step);
 
-    // Header file installation
-    b.installFile("include/proven.h", "include/proven.h");
+    // Header file installation (optional - for C/C++ consumers only)
+    // The Zig library exposes a C ABI, but consumers using Zig, Rust (via zig-cc),
+    // or other modern FFI mechanisms don't need a header file.
+    // Uncomment if you need C header for legacy interop:
+    // b.installFile("include/proven.h", "include/proven.h");
 
     // Tests
     const main_tests = b.addTest(.{

ffi/zig/src/main.zig

@@ -922,7 +922,8 @@ export fn proven_header_render(
 }
 
 /// Build Content-Security-Policy header value from directives
-export fn proven_header_build_csp(directives_json: ?[*]const u8, json_len: usize) StringResult {
+export fn proven_header_build_csp(directives_json: ?[*]const u8, _json_len: usize) StringResult {
+    _ = _json_len; // Will be used when JSON parsing is implemented
     if (directives_json == null) {
         return .{ .status = .err_null_pointer, .value = null, .length = 0 };
     }