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.

cmd/wcc/err.c

@@ -0,0 +1,76 @@
+/*
+ * err.c — diagnostics.
+ *
+ * fatal	prints, sets exit(1).
+ * errorf	prints with source location, increments nerrors.
+ * warnf	prints with source location, increments nwarnings.
+ *
+ * Plan 9 style: short, no levels beyond fatal/error/warn, no colour.
+ */
+#include "ww.h"
+#include <stdlib.h>
+#include <string.h>
+
+Pos	noPos = { "<none>", 0, 0 };
+int	nerrors;
+int	nwarnings;
+FILE	*errout;	/* set by main; defaults to stderr */
+
+static FILE *
+out(void)
+{
+	return errout ? errout : stderr;
+}
+
+static void
+prefix(Pos p)
+{
+	FILE *f = out();
+	if (p.file == NULL)
+		p = noPos;
+	if (p.line > 0)
+		fprintf(f, "%s:%d:%d: ", p.file, p.line, p.col);
+	else
+		fprintf(f, "%s: ", p.file);
+}
+
+void
+fatal(const char *fmt, ...)
+{
+	FILE *f = out();
+	va_list ap;
+	fprintf(f, "ww: ");
+	va_start(ap, fmt);
+	vfprintf(f, fmt, ap);
+	va_end(ap);
+	fprintf(f, "\n");
+	exit(1);
+}
+
+void
+errorf(Pos p, const char *fmt, ...)
+{
+	FILE *f = out();
+	va_list ap;
+	prefix(p);
+	fprintf(f, "error: ");
+	va_start(ap, fmt);
+	vfprintf(f, fmt, ap);
+	va_end(ap);
+	fprintf(f, "\n");
+	nerrors++;
+}
+
+void
+warnf(Pos p, const char *fmt, ...)
+{
+	FILE *f = out();
+	va_list ap;
+	prefix(p);
+	fprintf(f, "warning: ");
+	va_start(ap, fmt);
+	vfprintf(f, fmt, ap);
+	va_end(ap);
+	fprintf(f, "\n");
+	nwarnings++;
+}