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/991_w6a_ww.c

@@ -0,0 +1,135 @@
+/*
+ * 991_w6a_ww — phase-10 marker for the ww-side w6a port.
+ *
+ * Diffs the C-built `w6a` against the ww-built `w6a_ww` on a corpus of
+ * .s files. Both must produce byte-identical ELF .o output. The corpus
+ * spans:
+ *   - rt/X.s              hand-written runtime; small, varied operands
+ *   - generated .s        from the existing selfhost cgen pipeline
+ *                         (wwdump/w6a/w6l main.s) - large, exercises
+ *                         every encoding path w6c emits
+ *
+ * Any divergence is a port bug in selfhost/cmd/w6a/.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.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;
+}
+
+static const char *
+absbin(void)
+{
+	const char *b = getenv("BIN");
+	if (!b) b = "out/bin";
+	if (b[0] == '/') return b;
+	static char buf[2048];
+	char cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return NULL;
+	snprintf(buf, sizeof buf, "%s/%s", cwd, b);
+	return buf;
+}
+
+static int
+slurp(const char *path, char **outbuf, size_t *outlen)
+{
+	FILE *f = fopen(path, "rb");
+	if (!f) return -1;
+	fseek(f, 0, SEEK_END);
+	long n = ftell(f);
+	fseek(f, 0, SEEK_SET);
+	if (n < 0) { fclose(f); return -1; }
+	char *b = malloc((size_t)n + 1);
+	if (!b) { fclose(f); return -1; }
+	if (fread(b, 1, (size_t)n, f) != (size_t)n) { free(b); fclose(f); return -1; }
+	b[n] = '\0';
+	fclose(f);
+	*outbuf = b;
+	*outlen = (size_t)n;
+	return 0;
+}
+
+static int
+diff_one(const char *bin, const char *src)
+{
+	char co[64], wo[64], cmd[2048];
+	snprintf(co, sizeof co, "/tmp/wwa_%d_c.o", getpid());
+	snprintf(wo, sizeof wo, "/tmp/wwa_%d_w.o", getpid());
+
+	snprintf(cmd, sizeof cmd, "%s/w6a -o %s %s 2>/dev/null", bin, co, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "w6a_ww FAIL: C w6a errored on %s\n", src);
+		unlink(co);
+		return -1;
+	}
+	snprintf(cmd, sizeof cmd, "%s/w6a_ww -o %s %s 2>/dev/null", bin, wo, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "w6a_ww FAIL: ww w6a errored on %s\n", src);
+		unlink(co); unlink(wo);
+		return -1;
+	}
+	char *bc = NULL, *bw = NULL;
+	size_t nc = 0, nw = 0;
+	int rc = 0;
+	if (slurp(co, &bc, &nc) < 0 || slurp(wo, &bw, &nw) < 0) {
+		fprintf(stderr, "w6a_ww FAIL: cannot read .o for %s\n", src);
+		rc = -1;
+	} else if (nc != nw || memcmp(bc, bw, nc) != 0) {
+		fprintf(stderr, "w6a_ww FAIL: %s — C %zu vs ww %zu bytes\n",
+		    src, nc, nw);
+		rc = -1;
+	}
+	free(bc); free(bw);
+	unlink(co); unlink(wo);
+	return rc;
+}
+
+int
+main(void)
+{
+	const char *bin = absbin();
+	if (!bin) return 1;
+
+	char cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+
+	const char *files[] = {
+		/* hand-written runtime — small, covers MOVQ/LEAQ/CALL/SYSCALL/RET. */
+		"rt/start.s",
+		"rt/syscall.s",
+		"rt/abort.s",
+		"rt/alloc.s",
+		"rt/streq.s",
+		/* generated .s from the existing selfhost stack — large,
+		 * exercises every encoding w6c emits (the only way to hit
+		 * the long tail of operand shapes). */
+		"selfhost/cmd/wwdump/main.s",
+		"selfhost/cmd/w6l/main.s",
+		"selfhost/cmd/w6a/main.s",
+		NULL,
+	};
+	int fail = 0;
+	int n = 0;
+	for (int i = 0; files[i]; i++) {
+		char p[2048];
+		snprintf(p, sizeof p, "%s/%s", cwd, files[i]);
+		if (diff_one(bin, p) != 0) fail++;
+		n++;
+	}
+	if (fail) {
+		fprintf(stderr, "w6a_ww: %d/%d diff(s) failed\n", fail, n);
+		return 1;
+	}
+	printf("w6a_ww: byte-identical to C w6a on %d corpus files "
+	    "(rt/*.s + selfhost generated main.s)\n", n);
+	return 0;
+}