dir.c: cache and revalidate working directory

`rb_dir_getwd_ospath()` is called quite frequently, but
the overwhelming majority of the time, the current directory
didn't change.

We can also assume that most of the time, `PATH_MAX` is enough
for `getcwd`, hence we can first attempt to use a small stack
buffer rather than always allocate on the heap.

This way we can keep the last `pwd` and revalidate it with no
allocation.

On macOS syscalls are fairly slow, so the gain isn't very large.

macOS:

```
compare-ruby: ruby 4.1.0dev (2026-04-09T05:19:02Z master c091c18) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-04-09T06:37:20Z get-cwd-cache ea02126d79) +PRISM [arm64-darwin25]
```

|         |compare-ruby|built-ruby|
|:--------|-----------:|---------:|
|Dir.pwd  |    105.183k|  113.420k|
|         |           -|     1.08x|
```

Linux (inside virtualized Docker)

```
compare-ruby: ruby 4.1.0dev (2026-04-07T08:26:25Z master fcd2100) +PRISM [aarch64-linux]
built-ruby: ruby 4.1.0dev (2026-04-09T06:38:09Z get-cwd-cache 6774af9) +PRISM [aarch64-linux]
```

|         |compare-ruby|built-ruby|
|:--------|-----------:|---------:|
|Dir.pwd  |      4.157M|    5.541M|
|         |           -|     1.33x|

Author: byroot

benchmark/dir_pwd.yml

@@ -0,0 +1,2 @@
+benchmark:
+  pwd: Dir.pwd

dir.c

@@ -1585,6 +1585,8 @@ dir_chdir(VALUE dir)
 #endif
 }
 
+static VALUE last_cwd;
+
 #ifndef _WIN32
 static VALUE
 getcwd_to_str(VALUE arg)
@@ -1604,20 +1606,43 @@ getcwd_xfree(VALUE arg)
     return Qnil;
 }
 
-VALUE
-rb_dir_getwd_ospath(void)
+static VALUE
+rb_dir_getwd_ospath_slowpath(void)
 {
     char *path = ruby_getcwd();
     return rb_ensure(getcwd_to_str, (VALUE)path, getcwd_xfree, (VALUE)path);
 }
+
+VALUE
+rb_dir_getwd_ospath(void)
+{
+    char buf[PATH_MAX];
+    char *path = getcwd(buf, PATH_MAX);
+    if (!path) {
+        return rb_dir_getwd_ospath_slowpath();
+    }
+
+    VALUE cached_cwd = RUBY_ATOMIC_VALUE_LOAD(last_cwd);
+
+    if (!cached_cwd || strcmp(RSTRING_PTR(cached_cwd), path) != 0) {
+#ifdef __APPLE__
+        cached_cwd = rb_str_normalize_ospath(path, strlen(path));
+#else
+        cached_cwd = rb_str_new2(path);
+#endif
+        rb_str_freeze(cached_cwd);
+        RUBY_ATOMIC_VALUE_SET(last_cwd, cached_cwd);
+    }
+    return cached_cwd;
+}
 #endif
 
 VALUE
 rb_dir_getwd(void)
 {
     rb_encoding *fs = rb_filesystem_encoding();
     int fsenc = rb_enc_to_index(fs);
-    VALUE cwd = rb_dir_getwd_ospath();
+    VALUE cwd = rb_str_new_shared(rb_dir_getwd_ospath());
 
     switch (fsenc) {
       case ENCINDEX_US_ASCII:
@@ -4008,6 +4033,7 @@ Init_Dir(void)
 
     rb_gc_register_address(&chdir_lock.path);
     rb_gc_register_address(&chdir_lock.thread);
+    rb_gc_register_address(&last_cwd);
 
     rb_cDir = rb_define_class("Dir", rb_cObject);