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/610_arch.c

@@ -0,0 +1,96 @@
+/*
+ * 610_arch — archive selectivity. Build two .o files into an archive
+ * where one defines a symbol main needs, and the other references an
+ * undefined external. Linking should pull only the needed member;
+ * the other one's bad reference must NOT cause a link error.
+ *
+ * If w6l were still pulling all members, this test would fail with
+ * "undefined reference to 'this_symbol_does_not_exist'".
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static int run(const char *cmd) { return system(cmd); }
+
+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 dir[64];
+	snprintf(dir, sizeof dir, "/tmp/wwarch_%d", getpid());
+	mkdir(dir, 0755);
+
+	/* lib_good.ww — defines f1() */
+	char path[256];
+	snprintf(path, sizeof path, "%s/lib_good.ww", dir);
+	FILE *f = fopen(path, "wb");
+	fputs("export fn f1() i32 = { return 7; };", f);
+	fclose(f);
+
+	/* lib_bad.ww — defines f2() but also references an undefined extern */
+	snprintf(path, sizeof path, "%s/lib_bad.ww", dir);
+	f = fopen(path, "wb");
+	fputs("@symbol(\"this_symbol_does_not_exist\") fn bogus() i32;\n"
+	      "export fn f2() i32 = { return bogus(); };", f);
+	fclose(f);
+
+	/* main.ww — calls f1, NOT f2 */
+	snprintf(path, sizeof path, "%s/m.ww", dir);
+	f = fopen(path, "wb");
+	fputs("@symbol(\"f1\") fn f1() i32;\n"
+	      "fn main() i32 = { return f1(); };", f);
+	fclose(f);
+
+	char cmd[2048];
+	snprintf(cmd, sizeof cmd,
+	    "set -e; cd %s && %s/w6c -o lib_good.s lib_good.ww && "
+	    "%s/w6c -o lib_bad.s lib_bad.ww && "
+	    "%s/w6a -o lib_good.o lib_good.s && %s/w6a -o lib_bad.o lib_bad.s && "
+	    "ar rcs libfoo.a lib_good.o lib_bad.o && "
+	    "%s/w6c -o m.s m.ww && %s/w6a -o m.o m.s",
+	    dir, bin, bin, bin, bin, bin, bin);
+	if (run(cmd) != 0) { fprintf(stderr, "build failed\n"); return 1; }
+
+	/* Find start.o for the runtime */
+	char startobj[256];
+	snprintf(startobj, sizeof startobj, "%s/../obj/rt/start.o", bin);
+
+	snprintf(cmd, sizeof cmd,
+	    "%s/w6l -o %s/m %s/m.o %s %s/libfoo.a 2>%s/link.err",
+	    bin, dir, dir, startobj, dir, dir);
+	if (run(cmd) != 0) {
+		FILE *ef = fopen("/dev/null", "r");
+		(void)ef;
+		char errpath[300];
+		snprintf(errpath, sizeof errpath, "%s/link.err", dir);
+		FILE *e = fopen(errpath, "r");
+		if (e) {
+			char buf[512]; size_t r = fread(buf, 1, sizeof buf - 1, e); buf[r]='\0'; fclose(e);
+			fprintf(stderr, "link failed:\n%s\n", buf);
+		}
+		return 1;
+	}
+
+	char exepath[256];
+	snprintf(exepath, sizeof exepath, "%s/m", dir);
+	int rc = run(exepath);
+	if (WIFEXITED(rc) && WEXITSTATUS(rc) == 7) {
+		printf("arch: ok (only needed archive member pulled)\n");
+		return 0;
+	}
+	fprintf(stderr, "arch: unexpected exit\n");
+	return 1;
+}