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/600_w6l.c

@@ -0,0 +1,67 @@
+/*
+ * 600_6l — linker smoke. Drive w6c → w6a → w6l on a tiny program,
+ * confirm the result is a static ELF executable with no PT_INTERP.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+
+	char src[64], asmf[64], obj[64], exe[64];
+	snprintf(src, sizeof src, "/tmp/wwt_%d.ww", getpid());
+	snprintf(asmf, sizeof asmf, "/tmp/wwt_%d.s", getpid());
+	snprintf(obj, sizeof obj, "/tmp/wwt_%d.o", getpid());
+	snprintf(exe, sizeof exe, "/tmp/wwt_%d.x", getpid());
+
+	FILE *f = fopen(src, "wb");
+	fputs("fn main() i32 = { return 42; };", f);
+	fclose(f);
+
+	char cmd[1024];
+	snprintf(cmd, sizeof cmd, "%s/w6c -o %s %s", bin, asmf, src);
+	if (system(cmd) != 0) { fprintf(stderr, "w6c failed\n"); return 1; }
+	snprintf(cmd, sizeof cmd, "%s/w6a -o %s %s", bin, obj, asmf);
+	if (system(cmd) != 0) { fprintf(stderr, "w6a failed\n"); return 1; }
+	snprintf(cmd, sizeof cmd, "%s/w6l -o %s %s", bin, exe, obj);
+	if (system(cmd) != 0) { fprintf(stderr, "w6l failed\n"); return 1; }
+
+	/* validate ELF magic + e_type=EXEC */
+	FILE *of = fopen(exe, "rb");
+	if (of == NULL) { fprintf(stderr, "exe missing\n"); return 1; }
+	unsigned char hdr[20];
+	if (fread(hdr, 1, sizeof hdr, of) != sizeof hdr) { fprintf(stderr, "short exe\n"); fclose(of); return 1; }
+	fclose(of);
+
+	if (memcmp(hdr, "\x7f""ELF", 4) != 0 || hdr[4] != 2) {
+		fprintf(stderr, "not an ELF64\n"); return 1;
+	}
+	/* e_type at offset 16, little-endian u16; ET_EXEC = 2 */
+	unsigned short etype = (unsigned short)hdr[16] | ((unsigned short)hdr[17] << 8);
+	if (etype != 2) {
+		fprintf(stderr, "not ET_EXEC, got %u\n", etype); return 1;
+	}
+
+	/* ldd "not a dynamic executable" — proxy: check that file is not
+	 * dynamic by looking for PT_INTERP. We have none, so ldd reports
+	 * "not a dynamic executable" or similar. */
+	snprintf(cmd, sizeof cmd, "ldd %s 2>&1", exe);
+	FILE *p = popen(cmd, "r");
+	char buf[256] = {0};
+	fread(buf, 1, sizeof buf - 1, p);
+	pclose(p);
+	if (strstr(buf, "not a dynamic executable") == NULL
+	    && strstr(buf, "statically linked") == NULL) {
+		fprintf(stderr, "ldd says not static: %s\n", buf);
+		return 1;
+	}
+
+	unlink(src); unlink(asmf); unlink(obj); unlink(exe);
+	printf("w6l: ok\n");
+	return 0;
+}