windows: address review feedback on DLL CRT init PR

Use b.allocator instead of b.graph.arena for SDK detection and
path formatting -- b.allocator is the public API, b.graph.arena
is an internal field.

Move test_dll_init.c from windows/Ghostty.Tests/ to test/windows/
with a README. Test infrastructure belongs under test/, not the
Windows app directory.

src/build/GhosttyLib.zig

@@ -109,7 +109,7 @@ pub fn initShared(
         // ucrt.lib is in the Windows SDK 'ucrt' dir. Detect the SDK
         // installation and add the UCRT library path.
         const arch = deps.config.target.result.cpu.arch;
-        const sdk = std.zig.WindowsSdk.find(b.graph.arena, arch) catch null;
+        const sdk = std.zig.WindowsSdk.find(b.allocator, arch) catch null;
         if (sdk) |s| {
             if (s.windows10sdk) |w10| {
                 const arch_str: []const u8 = switch (arch) {
@@ -119,7 +119,7 @@ pub fn initShared(
                     else => "x64",
                 };
                 const ucrt_lib_path = std.fmt.allocPrint(
-                    b.graph.arena,
+                    b.allocator,
                     "{s}\\Lib\\{s}\\ucrt\\{s}",
                     .{ w10.path, w10.version, arch_str },
                 ) catch null;

test/windows/README.md

@@ -0,0 +1,28 @@
+# Windows Tests
+
+Manual test programs for Windows-specific functionality.
+
+## test_dll_init.c
+
+Regression test for the DLL CRT initialization fix. Loads ghostty.dll
+at runtime and calls ghostty_info + ghostty_init to verify the MSVC C
+runtime is properly initialized.
+
+### Build
+
+```
+zig cc test_dll_init.c -o test_dll_init.exe -target native-native-msvc
+```
+
+### Run
+
+```
+copy ..\..\zig-out\lib\ghostty.dll . && test_dll_init.exe
+```
+
+Expected output (after the CRT fix):
+
+```
+ghostty_info: <version string>
+ghostty_init: 0
+```