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.

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
+```

test/windows/test_dll_init.c

@@ -0,0 +1,54 @@
+/*
+ * Minimal reproducer for the libghostty DLL CRT initialization issue.
+ *
+ * Before the fix (DllMain calling __vcrt_initialize / __acrt_initialize),
+ * ghostty_init crashed with "access violation writing 0x0000000000000024"
+ * because Zig's _DllMainCRTStartup does not initialize the MSVC C runtime
+ * for DLL targets.
+ *
+ * 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 fix):
+ *   ghostty_info: <version string>
+ *   ghostty_init: 0
+ */
+
+#include <stdio.h>
+#include <windows.h>
+
+typedef struct {
+    int build_mode;
+    const char *version;
+    size_t version_len;
+} ghostty_info_s;
+
+typedef ghostty_info_s (*ghostty_info_fn)(void);
+typedef int (*ghostty_init_fn)(size_t, char **);
+
+int main(void) {
+    HMODULE dll = LoadLibraryA("ghostty.dll");
+    if (!dll) {
+        fprintf(stderr, "LoadLibrary failed: %lu\n", GetLastError());
+        return 1;
+    }
+
+    ghostty_info_fn info_fn = (ghostty_info_fn)GetProcAddress(dll, "ghostty_info");
+    if (info_fn) {
+        ghostty_info_s info = info_fn();
+        fprintf(stderr, "ghostty_info: %.*s\n", (int)info.version_len, info.version);
+    }
+
+    ghostty_init_fn init_fn = (ghostty_init_fn)GetProcAddress(dll, "ghostty_init");
+    if (init_fn) {
+        char *argv[] = {"ghostty"};
+        int result = init_fn(1, argv);
+        fprintf(stderr, "ghostty_init: %d\n", result);
+        if (result != 0) return 1;
+    }
+
+    /* Skip FreeLibrary -- ghostty's global state cleanup and CRT
+     * teardown ordering is not yet handled. The OS reclaims everything
+     * on process exit. */
+    return 0;
+}