lib+test: add log over io.stream sink

Hare-shaped lib/log v1: logger vtable carrying one println slot,
stdlogger forwarding to a *io.stream, plus *logger globals (silent
/ default / global), setlogger, lprintln / println, lfatal / fatal.
Default sink is stderr through a private rt_syscall write callback;
graduates with #17 (cgen mod-mangle for fn labels), at which point
the rt_syscall stub disappears the same way fmt's will.

Module-scope struct/pointer-literal init isn't constexpr in cstage
emit_lets, so silent/default/global are wired lazily by ensureinit
on the first exported-fn entry (lib/temp rnginit pattern). Callers
that read the globals directly must call some lib/log fn first.

Skipped this round: printfln / lprintfln / fatalf / lfatalf and the
matching printfln vtable slot — they need a fmt {n}-placeholder
parser that isn't shipped yet. fatal / lfatal's exit(255) arm has
no test fixture (needs fork+wait for WEXITSTATUS); left as TODO.

971_log_run wraps the @test fixture under `ww run`, mirroring
970_fmt_run. make test: 54/54; bootstrap fixed-point (990-997)
holds.

test/wcc/971_log_run.c

@@ -0,0 +1,52 @@
+/*
+ * 971_log_run — execute the lib/log @test fixture under the C-side
+ * `ww run` driver and assert exit 0.
+ *
+ * log sits at the same tier as fmt (both consume io.stream sinks);
+ * slotting at 971, adjacent to 970_fmt_run, keeps both stdlib-runtime
+ * tests ahead of the 980-989 sub-band. logtest.ww carries its own
+ * `export fn main()` that drives the @test fns and signals which
+ * case failed via the exit code, so this file is a thin wrapper —
+ * no @test scanning, no synthetic main generation.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return 1;
+}
+
+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 cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+
+	const char *src = "lib/log/logtest.ww";
+	char path[1024], cmd[2048];
+	snprintf(path, sizeof path, "%s/%s", cwd, src);
+	snprintf(cmd, sizeof cmd, "%s/ww run %s", bin, path);
+	int rc = runwait(cmd);
+	if (rc != 0) {
+		fprintf(stderr, "log_run FAIL: %s exited %d\n", src, rc);
+		return 1;
+	}
+	printf("log_run: %s ok\n", src);
+	return 0;
+}