ffi #jni_main: emit static { System.loadLibrary(...); } in the Java class

Required for Android to resolve the `Java_*` symbols R.3 synthesises:
without `System.loadLibrary(...)` running before the Activity calls its
first native method, JNI lookup fails with UnsatisfiedLinkError.

The lib name comes from the build's `-o` basename — `/tmp/libsxchess.so`
→ `sxchess` — derived in `Compilation.collectJniMainEmissions` and
threaded through new `jni_java_emit.Options.lib_name`. When `-o` is
unset (or doesn't match `lib*.so`), the emitter omits the static init
and the caller must arrange loading another way.

dex confirmation on the slice 2 smoke: `<clinit>` static constructor
appears alongside `<init>` and `sx_onCreate` — the bytecode invokes
`System.loadLibrary("sxjnimain")` matching `/tmp/libsxjnimain.so`.

131 host / 4 cross / zig build test all green.

src/core.zig

@@ -244,6 +244,12 @@ pub const Compilation = struct {
             try registry.put(entry.key_ptr.*, entry.value_ptr.*.foreign_path);
         }
 
+        // Derive the `System.loadLibrary` argument from the `-o` basename
+        // (e.g. `/tmp/libsxchess.so` → `sxchess`). When `-o` is unset the
+        // emitter omits the static init block; the user must then arrange
+        // .so loading via another class.
+        const lib_name = libNameFromOutputPath(self.target_config.output_path);
+
         var it = lowering.foreign_class_map.iterator();
         while (it.next()) |entry| {
             const fcd = entry.value_ptr.*;
@@ -253,7 +259,10 @@ pub const Compilation = struct {
             if (seen.contains(fcd.foreign_path)) continue;
             try seen.put(fcd.foreign_path, {});
 
-            const java_source = try ir.jni_java_emit.emitJavaSource(self.allocator, fcd, .{ .classes = &registry });
+            const java_source = try ir.jni_java_emit.emitJavaSource(self.allocator, fcd, .{
+                .classes = &registry,
+                .lib_name = lib_name,
+            });
             try self.lowering_jni_main_decls.append(self.allocator, .{
                 .foreign_path = try self.allocator.dupe(u8, fcd.foreign_path),
                 .java_source = java_source,
@@ -261,6 +270,16 @@ pub const Compilation = struct {
         }
     }
 
+    /// `/path/to/libfoo.so` → `foo`. Anything else → null (caller skips
+    /// emitting the `System.loadLibrary` init block).
+    fn libNameFromOutputPath(output_path: ?[]const u8) ?[]const u8 {
+        const path = output_path orelse return null;
+        const basename = std.fs.path.basename(path);
+        if (!std.mem.startsWith(u8, basename, "lib")) return null;
+        if (!std.mem.endsWith(u8, basename, ".so")) return null;
+        return basename[3 .. basename.len - 3];
+    }
+
     /// Java sources rendered from `#jni_main #jni_class("...")` decls during
     /// lowering. Empty unless `lowerToIR` has run.
     pub fn getJniMainEmissions(self: *const Compilation) []const JniMainEmission {