android: forward NDK sysroot to embedded clang + skip auto #library/#framework

C imports (stb_image, stb_truetype) compiled via the embedded LLVM
clang library now resolve bionic headers on Android. main.zig auto-
fills target_config.sysroot with the NDK root (mirroring the iOS path
that auto-fills the iOS SDK path); c_import.zig derives the bionic
sysroot inside it and passes `--sysroot <ndk>/toolchains/llvm/prebuilt/<host>/sysroot`
to the embedded clang.

Android link branch in target.zig stops auto-appending entries from the
collected `#library` / `#framework` lists. Most `#library` directives
in the stdlib (`objc.sx`'s `objc :: #library "objc";`, frameworks set
by uikit.sx, etc.) describe Apple-specific intent that's nonsensical on
Android. Users opt into Android-side libs via `opts.add_link_flag(...)`
in build.sx — same shape as how the iOS branch already lists frameworks
in chess's configure_build.

Verified end-to-end: chess game compiles for Android, packages into a
debug-signed APK, installs and launches on Pixel 7 Pro. It crashes in
UIRenderer.init because raw GL calls run before EGL is up — same
architectural gap iOS bridges via the Metal GPU protocol. Next step
is a GLES3 GPU impl or a lazy-init in UIRenderer. 86/86 regression
tests + iOS-sim chess build clean.

src/c_import.zig

@@ -10,6 +10,15 @@ pub const CSourceLocation = struct {
     line: u32,
 };
 
+/// Derive the NDK sysroot path from the NDK root (which by convention
+/// lives in `target_config.sysroot` on Android — see target.zig's
+/// Android link branch + main.zig's auto-discovery). Returns a NUL-
+/// terminated path suitable for clang's `--sysroot <path>` argv.
+fn androidSysrootFromNdkRoot(allocator: std.mem.Allocator, ndk_root: []const u8) ![:0]u8 {
+    const host_tag: []const u8 = if (builtin.os.tag == .macos) "darwin-x86_64" else "linux-x86_64";
+    return try std.fmt.allocPrintSentinel(allocator, "{s}/toolchains/llvm/prebuilt/{s}/sysroot", .{ ndk_root, host_tag }, 0);
+}
+
 pub const CImportResult = struct {
     fn_decls: []const *Node,
     /// Source locations for each fn_decl (parallel array, same indices).
@@ -193,6 +202,18 @@ pub fn compileCToObjects(
             try args_list.append(allocator, "-isysroot");
             try args_list.append(allocator, (try allocator.dupeZ(u8, sr)).ptr);
         }
+        // Android: route through the NDK sysroot so bionic headers resolve.
+        // The embedded clang library doesn't know how to be an Android cross-
+        // compiler on its own. `target_config.sysroot` holds the NDK root
+        // by convention (main.zig auto-fills it for --target android), so
+        // derive the headers/libs sysroot inside it.
+        if (target_config.isAndroid()) {
+            if (target_config.sysroot) |ndk_root| {
+                const sysroot = try androidSysrootFromNdkRoot(allocator, ndk_root);
+                try args_list.append(allocator, "--sysroot");
+                try args_list.append(allocator, sysroot.ptr);
+            }
+        }
         for (info.includes) |inc| {
             const dir = dirName(inc);
             try args_list.append(allocator, (try allocPrintZ(allocator, "-I{s}", .{dir})).ptr);