ww: rename toolchain to w-prefix + hare-style build/run/test driver

Plan 9-style w-prefix on the per-arch tools, disambiguating from the
real Plan 9 6c/6a/6l in ref/plan9front/:

    cmd/wwc/      → cmd/wcc/        libwwc.a → libwcc.a
    cmd/6{c,a,l}  → cmd/w6{c,a,l}   binary names too
    test/wwc/     → test/wcc/       6 test files w/ w6 prefix
    selfhost/cmd  mirror in lockstep
    bootstrap/amd64/{w6c,w6a,w6l}   snapshot binaries (gitignored)
    WW_6{C,A,L}   → WW_W6{C,A,L}    env-var overrides

Plan 9 source-tree refs ("Plan 9 6c shape", ref/plan9front/, etc.)
preserved. Hare-style driver, both C and ww sides:

    ww test [path]   discover *_test.ww in a directory module, run
                     each; single-file mode for `ww test foo.ww`
    Module-by-name   `ww build foo` resolves to foo.ww or foo/foo.ww
                     via search path (cwd : -I dirs : $WW_LIB)
    Default-to-cwd   `ww build` / `ww test` build the cwd module
    Run pass-through `ww run path arg1 arg2` reaches the program

lib/os: getcwd (79) and getdents64 (217) syscalls power `.` resolution
and directory enumeration on the ww side.

Makefile: wwstage tool deps now include lib/os/os.ww (+ lib/strconv
for wwdump_ww) so lib/* edits force their rebuild instead of leaving
stale binaries — surfaced when test 995 first failed against a stale
w6c_ww built before the lib/os additions.

Test 993 byte-identical parity gate (C-side ww vs ww-side ww_ww on a
build corpus) stays green; all 19 tests pass.

test/wcc/900_stdlib.c

@@ -0,0 +1,65 @@
+/*
+ * 900_stdlib — verify each stdlib module parses, type-checks, and
+ * codegens. We don't link or run them; they exist for downstream
+ * users to import. As real applications come online they will start
+ * exercising the bodies through the e2e harness.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static const char *modules[] = {
+	"lib/types/types.ww",
+	"lib/ascii/ascii.ww",
+	"lib/bytes/bytes.ww",
+	"lib/strings/strings.ww",
+	"lib/io/stream.ww",
+	"lib/errors/errors.ww",
+	"lib/os/os.ww",
+	"lib/strconv/strconv.ww",
+	"lib/sort/sort.ww",
+	"lib/path/path.ww",
+	"lib/encoding/utf8/utf8.ww",
+	"lib/encoding/hex/hex.ww",
+	"lib/hash/fnv/fnv.ww",
+	"lib/time/time.ww",
+	"lib/c/libc/libc.ww",
+	"lib/bufio/bufio.ww",
+	"lib/fmt/fmt.ww",
+	"lib/net/net.ww",
+	"lib/slices/slices.ww",
+	NULL
+};
+
+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;
+
+	int n = 0, fail = 0;
+	for (int i = 0; modules[i]; i++, n++) {
+		char path[1024], cmd[2048];
+		snprintf(path, sizeof path, "%s/%s", cwd, modules[i]);
+		snprintf(cmd, sizeof cmd, "%s/w6c %s > /dev/null 2>&1",
+		    bin, path);
+		int rc = system(cmd);
+		if (rc != 0) {
+			fprintf(stderr, "stdlib FAIL: %s (rc=%d)\n", modules[i], rc);
+			fail++;
+		}
+	}
+	if (fail) { fprintf(stderr, "%d/%d stdlib modules failed\n", fail, n); return 1; }
+	printf("stdlib: %d/%d ok\n", n, n);
+	return 0;
+}