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/800_ffi.c

@@ -0,0 +1,80 @@
+/*
+ * 800_ffi — verify the @symbol("X") attribute redirects CALLs at
+ * the codegen layer. The ww source declares a body-less fn with an
+ * external symbol name, calls it; the emitted .s must reference the
+ * external name, not the ww-side ident.
+ *
+ * Also smoke-tests building lib/c/libc/libc.ww (parse + check + cgen
+ * succeed; nothing to run).
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static char *
+run6c(const char *bin, const char *src)
+{
+	char path[64];
+	snprintf(path, sizeof path, "/tmp/wwffi_%d.ww", getpid());
+	FILE *f = fopen(path, "wb");
+	fputs(src, f);
+	fclose(f);
+
+	char cmd[512];
+	snprintf(cmd, sizeof cmd, "%s/w6c %s 2>&1", bin, path);
+	FILE *p = popen(cmd, "r");
+	size_t cap = 4096, n = 0;
+	char *buf = malloc(cap);
+	int c;
+	while ((c = fgetc(p)) != EOF) {
+		if (n + 1 >= cap) { cap *= 2; buf = realloc(buf, cap); }
+		buf[n++] = (char)c;
+	}
+	buf[n] = '\0';
+	pclose(p);
+	unlink(path);
+	return buf;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+
+	int fail = 0;
+
+	/* @symbol redirects ident name to symbol name */
+	{
+		char *out = run6c(bin,
+		    "@symbol(\"cout\") fn writeit(b: *u8, n: u64) i64;\n"
+		    "fn main() i32 = { writeit(nil, 0); return 0; };");
+		if (strstr(out, "CALL\tcout(SB)") == NULL) {
+			fprintf(stderr, "ffi: missing CALL cout(SB) in:\n%s\n", out);
+			fail++;
+		}
+		if (strstr(out, "CALL\twriteit(SB)") != NULL) {
+			fprintf(stderr, "ffi: should not call by ident name:\n%s\n", out);
+			fail++;
+		}
+		free(out);
+	}
+
+	/* lib/c/libc parses + cgens (no defined fns means no TEXT directives) */
+	{
+		char path[256];
+		snprintf(path, sizeof path, "%s/../../lib/c/libc/libc.ww", bin);
+		char cmd[512];
+		snprintf(cmd, sizeof cmd, "%s/w6c %s > /dev/null 2>&1", bin, path);
+		int rc = system(cmd);
+		if (rc != 0) {
+			fprintf(stderr, "ffi: w6c failed on libc.ww (rc=%d)\n", rc);
+			fail++;
+		}
+	}
+
+	if (fail) return 1;
+	puts("ffi: ok");
+	return 0;
+}