lib/os+test: add stat / lstat / fstat / exists

Hare-shaped filestat introspection. New types: filestat (80B,
mirrors fs::filestat ref/hare/fs/types.ha:141), mode (31-member
enum mirroring fs::mode ref/hare/fs/types.ha:63), stat_mask (7 bits
mirroring fs::stat_mask ref/hare/fs/types.ha:129), timespec (i64+i64,
layout-compatible with future lib/time::instant).

APIs: stat / lstat / fstat (*filestat, *u8|i32) (void|oserror) over
SYS_newfstatat (nr=262). The out-param shape sidesteps the cgreturn
24B ABI cap; commented inline. exists(*u8) bool goes through the
syscall directly rather than wrapping stat()? — dodges task #22's
80B-scrutinee match-slot disagreement until that lands.

Three latent cgen workarounds in tree, all pointer'd to filed tasks:
  #22: os.exists sidesteps the (void|oserror) match shape
  #24: `at` enum bundles AT_FDCWD/SYMLINK_NOFOLLOW/EMPTY_PATH instead
       of three top-level `def`s (negative-literal def DATA omit)
  #25: kstat.mode typed as `mode` (enum) rather than u32 to skip the
       redundant u32→enum cast emit

Tests: 976_stat_run, 9 rows — stat/lstat/fstat × regfile/dir/symlink
plus exists × {regfile,dir,noent}. Row 1 also pins perm-bit and
atime/mtime/ctime!=0 to catch silent kstat→filestat offset miscompiles
(kstat fields at 72/88/104).

Graduation to lib/fs when it ships is noted inline; signatures stay
rename-compatible.

test/wcc/976_stat_run.c

@@ -0,0 +1,107 @@
+/*
+ * 976_stat_run — execute the lib/os stat fixture under the C-side
+ * `ww run` driver and assert exit 0.
+ *
+ * Workflow (mirrors 975_dirs_run's mkdtemp + setenv + rm-rf shape):
+ *   1. mkdtemp /tmp/wwstat-XXXXXX — fresh per run, no parallel-test
+ *      contention.
+ *   2. Populate the scratch tree:
+ *        <tmp>/regfile      regular file, 11 bytes "hello world"
+ *        <tmp>/symlink   →  ./regfile  (relative symlink)
+ *        <tmp>/subdir       directory, mode 0700
+ *   3. setenv WW_TEST_STAT_REGFILE / SYMLINK / SUBDIR / NOENT with
+ *      the absolute paths; lib/os/stattest.ww reads them via
+ *      os.getenv and exercises stat/lstat/fstat/exists.
+ *   4. exec `ww run lib/os/stattest.ww`.
+ *   5. system("rm -rf <tmp>") regardless of pass/fail.
+ *
+ * Same wrapper shape as 970-975; cleanup-on-failure is rm-rf'd so a
+ * regression never leaves residue under /tmp.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return 1;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+	char cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+
+	char tmpl[64];
+	strcpy(tmpl, "/tmp/wwstat-XXXXXX");
+	if (mkdtemp(tmpl) == NULL) {
+		perror("mkdtemp");
+		return 1;
+	}
+
+	char regfile[256], symlink_path[256], subdir[256], noent[256];
+	snprintf(regfile, sizeof regfile, "%s/regfile", tmpl);
+	snprintf(symlink_path, sizeof symlink_path, "%s/symlink", tmpl);
+	snprintf(subdir, sizeof subdir, "%s/subdir", tmpl);
+	snprintf(noent, sizeof noent, "%s/does-not-exist", tmpl);
+
+	/* Populate. */
+	int fd = open(regfile, O_WRONLY | O_CREAT | O_EXCL, 0644);
+	if (fd < 0) { perror("open regfile"); goto cleanup_fail; }
+	if (write(fd, "hello world", 11) != 11) {
+		perror("write regfile"); close(fd); goto cleanup_fail;
+	}
+	close(fd);
+	if (symlink("./regfile", symlink_path) != 0) {
+		perror("symlink"); goto cleanup_fail;
+	}
+	if (mkdir(subdir, 0700) != 0) {
+		perror("mkdir subdir"); goto cleanup_fail;
+	}
+
+	setenv("WW_TEST_STAT_REGFILE", regfile, 1);
+	setenv("WW_TEST_STAT_SYMLINK", symlink_path, 1);
+	setenv("WW_TEST_STAT_SUBDIR", subdir, 1);
+	setenv("WW_TEST_STAT_NOENT", noent, 1);
+
+	const char *src = "lib/os/stattest.ww";
+	char path[1024], cmd[2048];
+	snprintf(path, sizeof path, "%s/%s", cwd, src);
+	snprintf(cmd, sizeof cmd, "%s/ww run %s", bin, path);
+	int rc = runwait(cmd);
+
+	char rmcmd[256];
+	snprintf(rmcmd, sizeof rmcmd, "rm -rf %s", tmpl);
+	(void)system(rmcmd);
+
+	if (rc != 0) {
+		fprintf(stderr, "stat_run FAIL: %s exited %d\n", src, rc);
+		return 1;
+	}
+	printf("stat_run: %s ok\n", src);
+	return 0;
+
+cleanup_fail: {
+	char rmcmd[256];
+	snprintf(rmcmd, sizeof rmcmd, "rm -rf %s", tmpl);
+	(void)system(rmcmd);
+	return 1;
+} }