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.

selfhost/cmd/wcc/err.ww

@@ -0,0 +1,37 @@
+// selfhost/cmd/wcc/err.ww — port of cmd/wcc/err.c.
+//
+// Diagnostics. Plan 9 style: short, no levels beyond fatal/error/warn.
+// Output goes through os.write so we don't pull in libc stdio.
+
+use os;
+use fmt;
+
+type pos = struct {
+	file: str,
+	line: i32,
+	col:  i32,
+};
+
+let nerrors:   i32 = 0;
+let nwarnings: i32 = 0;
+
+export fn fatal(msg: str) void = {
+	fmt.errln(msg);
+	os.exit(1);
+};
+
+export fn errorf(p: pos, msg: str) void = {
+	os.write(2, p.file.ptr, p.file.len: u64);
+	os.write(2, ": error: ".ptr, 9u64);
+	os.write(2, msg.ptr, msg.len: u64);
+	os.write(2, "\n".ptr, 1u64);
+	nerrors += 1;
+};
+
+export fn warnf(p: pos, msg: str) void = {
+	os.write(2, p.file.ptr, p.file.len: u64);
+	os.write(2, ": warning: ".ptr, 11u64);
+	os.write(2, msg.ptr, msg.len: u64);
+	os.write(2, "\n".ptr, 1u64);
+	nwarnings += 1;
+};