fonts: add tests for fallback font lookup w/ system ui font

...we're probably not supposed to be accessing system fonts this way.
when running the tests w/o my first fix (clearing restrictions for system font):
```
[I] ➜  ghostty git:(upstream-3) ✗ zig build test -Dtest-filter="discoverFallback"
test
└─ run ghostty-test 59/60 passed, 1 failed
error: 'font.discovery.test.coretext discoverFallback CJK codepoint' failed: 2025-09-10 13:53:31.792 ghostty-test[51978:12386835] CoreText note: Client requested name ".PingFangUITextSC-Regular", it will get TimesNewRomanPSMT rather than the intended font. All system UI font access should be through proper APIs such as CTFontCreateUIFontForLanguage() or +[NSFont systemFontOfSize:].
2025-09-10 13:53:31.792 ghostty-test[51978:12386835] CoreText note: Set a breakpoint on CTFontLogSystemFontNameRequest to debug.
/opt/homebrew/Cellar/zig/0.14.1/lib/zig/std/testing.zig:580:14: 0x10575e86f in expect (ghostty-test)
    if (!ok) return error.TestUnexpectedResult;
             ^
/Users/cheru/Code/ghostty/src/font/discovery.zig:1133:5: 0x10577445f in test.coretext discoverFallback CJK codepoint (ghostty-test)
    try testing.expect(face.hasCodepoint(0x4E2D, null));
    ^
error: while executing test 'os.TempDir.test_0', the following test command failed:
/Users/cheru/Code/ghostty/.zig-cache/o/d8ac3a9ccf171ecb22ca38e4e30bc420/ghostty-test --seed=0x9fe58469 --cache-dir=/Users/cheru/Code/ghostty/.zig-cache --listen=-
Build Summary: 49/51 steps succeeded; 1 failed; 59/60 tests passed; 1 failed
test transitive failure
└─ run ghostty-test 59/60 passed, 1 failed
error: the following build command failed with exit code 1:
/Users/cheru/Code/ghostty/.zig-cache/o/526a4ea34f8d95eb90ea2f3856ac80e8/build /opt/homebrew/Cellar/zig/0.14.1/bin/zig /opt/homebrew/Cellar/zig/0.14.1/lib/zig /Users/cheru/Code/ghostty /Users/cheru/Code/ghostty/.zig-cache /Users/cheru/.cache/zig --seed 0x9fe58469 -Z70fd58e5d3dc4a11 test -Dtest-filter=discoverFallback
```

Author: recursiveforte

src/font/discovery.zig

@@ -8,6 +8,8 @@ const opentype = @import("opentype.zig");
 const options = @import("main.zig").options;
 const Collection = @import("main.zig").Collection;
 const DeferredFace = @import("main.zig").DeferredFace;
+const Face = @import("main.zig").Face;
+const Library = @import("main.zig").Library;
 const Variation = @import("main.zig").face.Variation;
 
 const log = std.log.scoped(.discovery);
@@ -1053,3 +1055,77 @@ test "coretext sorting" {
         try testing.expectEqualStrings("SF Pro Bold Italic", name);
     }
 }
+
+test "coretext discoverFallback latin codepoint" {
+    if (options.backend != .coretext and options.backend != .coretext_freetype)
+        return error.SkipZigTest;
+
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var ct = CoreText.init();
+    defer ct.deinit();
+
+    var collection = Collection.init();
+    collection.load_options = .{
+        .library = try Library.init(alloc),
+    };
+    defer collection.deinit(alloc);
+
+    var disco_it = try ct.discover(alloc, .{ .family = ".AppleSystemUIFontMonospaced", .size = 12 });
+    defer disco_it.deinit();
+
+    const deferred_face = (try disco_it.next()).?;
+    _ = try collection.addDeferred(alloc, deferred_face, .{
+        .style = .regular,
+        .fallback = false,
+        .size_adjustment = .none,
+    });
+
+    // Test Latin codepoint (A)
+    var it = try ct.discoverFallback(alloc, &collection, .{
+        .codepoint = 'A',
+        .size = 12,
+    });
+    defer it.deinit();
+
+    const face = (try it.next()).?;
+    try testing.expect(face.hasCodepoint('A', null));
+}
+
+test "coretext discoverFallback CJK codepoint" {
+    if (options.backend != .coretext and options.backend != .coretext_freetype)
+        return error.SkipZigTest;
+
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var ct = CoreText.init();
+    defer ct.deinit();
+
+    var collection = Collection.init();
+    collection.load_options = .{
+        .library = try Library.init(alloc),
+    };
+    defer collection.deinit(alloc);
+
+    var disco_it = try ct.discover(alloc, .{ .family = ".AppleSystemUIFontMonospaced", .size = 12 });
+    defer disco_it.deinit();
+
+    const deferred_face = (try disco_it.next()).?;
+    _ = try collection.addDeferred(alloc, deferred_face, .{
+        .style = .regular,
+        .fallback = false,
+        .size_adjustment = .none,
+    });
+
+    // Test CJK codepoint (中)
+    var it = try ct.discoverFallback(alloc, &collection, .{
+        .codepoint = 0x4E2D,
+        .size = 12,
+    });
+    defer it.deinit();
+
+    const face = (try it.next()).?;
+    try testing.expect(face.hasCodepoint(0x4E2D, null));
+}