kernel: serialize test markers through the log lock; render one write per line

tests.zig wrote markers straight to serial, racing user-process records
rendered on other cores — visible as 16-byte UART-FIFO interleave once
the tagged renderer emitted several writes per record. Markers now go
through kernel log print (same serial sink, now under the log lock), the
renderer composes each line into one buffer and hits each sink once, and
the vfs-client-death needle drops the old self-written 'vfs: ' prefix.

system/kernel/log.zig

@@ -139,26 +139,45 @@ fn appendLocked(pid: u32, name: []const u8, level: abi.KlogLevel, now: u64, byte
 fn render(pid: u32, name: []const u8, level: abi.KlogLevel, line: []const u8, line_complete: bool) void {
     if (sink_count == 0) return;
     if (line.len == 0 and !line_complete) return;
+    // Compose the whole rendered piece first and emit it in ONE sink call per
+    // sink: fewer, larger UART writes, and no partial-line window should any
+    // path ever reach a sink without the log lock.
+    var buffer: [render_buffer_size]u8 = undefined;
+    var used: usize = 0;
     if (!at_line_start and open_line_pid != pid) {
-        fanOut("\n");
+        buffer[used] = '\n';
+        used += 1;
         at_line_start = true;
     }
     if (at_line_start and level != .raw) {
-        fanOut(name);
-        fanOut(": ");
-        switch (level) {
-            .err => fanOut("error: "),
-            .warn => fanOut("warning: "),
-            .debug => fanOut("debug: "),
-            .info, .raw => {},
-        }
+        used += place(buffer[used..], name);
+        used += place(buffer[used..], ": ");
+        used += place(buffer[used..], switch (level) {
+            .err => "error: ",
+            .warn => "warning: ",
+            .debug => "debug: ",
+            .info, .raw => "",
+        });
     }
-    fanOut(line);
-    if (line_complete) fanOut("\n");
+    used += place(buffer[used..], line);
+    if (line_complete and used < buffer.len) {
+        buffer[used] = '\n';
+        used += 1;
+    }
+    fanOut(buffer[0..used]);
     at_line_start = line_complete;
     open_line_pid = pid;
 }
 
+/// newline + name + ": warning: " + a full payload line + newline.
+const render_buffer_size = 1 + abi.maximum_process_name + 11 + abi.klog_maximum_message + 1;
+
+fn place(destination: []u8, bytes: []const u8) usize {
+    const n = @min(destination.len, bytes.len);
+    @memcpy(destination[0..n], bytes[0..n]);
+    return n;
+}
+
 fn fanOut(bytes: []const u8) void {
     for (sinks[0..sink_count]) |sink| sink(bytes);
 }