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/main.zig

@@ -167,6 +167,15 @@ pub fn main(init: std.process.Init) !void {
         target_config.sysroot = sx.target.discoverAppleSdk(allocator, io, sdk_name) catch null;
     }
 
+    // Same idea for Android — the NDK root must be visible to BOTH the
+    // C-import compile path (so `--sysroot ndk/.../sysroot` finds bionic
+    // headers) and the link path. By convention, target_config.sysroot
+    // holds the NDK root on Android (target.zig's link branch + c_import.zig
+    // both read it). Honors any explicit --sysroot.
+    if (target_config.isAndroid() and target_config.sysroot == null) {
+        target_config.sysroot = sx.target.discoverAndroidNdk(allocator, io) catch null;
+    }
+
     const path = input_path orelse {
         printUsage();
         return;
@@ -559,6 +568,7 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
         // Shadow the outer `fws` for the rest of the function by reassignment.
         fws = try merged_fws.toOwnedSlice(allocator);
     }
+
     if (build_flags.len > 0) {
         var all_flags: std.ArrayList([]const u8) = .empty;
         for (target_config.extra_link_flags) |f| try all_flags.append(allocator, f);