android target + APK pipeline; LSP imports honor stdlib paths

Android (toolchain):
  --target android / --target android-arm64 → aarch64-linux-android21.
  target.zig discovers $ANDROID_NDK_HOME (or scans
  ~/Library/Android/sdk/ndk/* for the newest), invokes the NDK clang
  with -shared -fPIC and links libsxhello.so against -llog -landroid
  -lEGL -lGLESv3 -lm -ldl. native_app_glue.c from the NDK is compiled
  and linked alongside the sx .o so apps can use the conventional
  android_main(struct android_app*) shape; -u ANativeActivity_onCreate
  keeps glue's symbol live.

Android (APK):
  --apk <out> wraps the .so into a debug-signed installable APK.
  target.zig discovers the SDK at $ANDROID_HOME (or
  ~/Library/Android/sdk), picks the newest build-tools + platforms,
  generates a NativeActivity AndroidManifest.xml from --bundle-id,
  packages via aapt2 link, appends the lib/ tree, zipalign, then
  apksigner against ~/.android/debug.keystore (auto-generated via
  keytool on first use). One command end-to-end:
      sx build --target android --apk out.apk \\
          --bundle-id co.swipelab.foo main.sx
  Verified on Pixel 7 Pro: install + launch reaches android_main.

Compiler (entry-point linkage):
  Top-level fn defs default to LLVM internal linkage and are lazily
  lowered (only `main` was eagerly lowered before). Added
  isExportedEntryName() — a small allowlist for names the OS loader
  calls: `main`, `android_main`, `ANativeActivity_onCreate`,
  `JNI_OnLoad`. These get eagerly lowered AND keep external linkage,
  so they actually land in .dynsym.

LSP (imports):
  DocumentStore now takes the install-discovered stdlib_paths and
  forwards them into resolveImportPath, mirroring the compiler. Before
  this, every `#import "modules/..."` resolved through the stdlib path
  failed silently inside the LSP and identifiers from those modules
  showed as `undefined variable`. Repro on label.sx: 1 false positive
  before, 0 after.

src/lsp/server.zig

@@ -24,13 +24,15 @@ pub const Server = struct {
     io: std.Io,
     shutdown_requested: bool = false,
     root_path: []const u8 = "",
+    stdlib_paths: []const []const u8 = &.{},
 
-    pub fn init(allocator: std.mem.Allocator, transport: *Transport, io: std.Io) Server {
+    pub fn init(allocator: std.mem.Allocator, transport: *Transport, io: std.Io, stdlib_paths: []const []const u8) Server {
         return .{
             .allocator = allocator,
-            .documents = DocumentStore.init(allocator, io),
+            .documents = DocumentStore.init(allocator, io, stdlib_paths),
             .transport = transport,
             .io = io,
+            .stdlib_paths = stdlib_paths,
         };
     }
 
@@ -271,7 +273,7 @@ pub const Server = struct {
         if (findImportPathAtOffset(doc.source, offset)) |import_path| {
             const base_dir = sx.imports.dirName(file_path);
             const rp: ?[]const u8 = if (self.root_path.len > 0) self.root_path else null;
-            const resolved = try sx.imports.resolveImportPath(self.allocator, self.io, base_dir, import_path, rp, &.{});
+            const resolved = try sx.imports.resolveImportPath(self.allocator, self.io, base_dir, import_path, rp, self.stdlib_paths);
 
             // For directory imports, try to read as file first
             if (std.Io.Dir.readFileAlloc(.cwd(), self.io, resolved, self.allocator, .limited(10 * 1024 * 1024))) |_| {