runtime: std.log for every user binary — kernel-stamped attribution

library/runtime/log.zig wires std.log to the tagged ring: the root shim
installs std_options for every binary (programs may override), logFn
formats one line per record and emits it with its level via
debug_write — the payload no longer carries the process's name; the
kernel stamps identity structurally and the serial renderer prints the
'<binary path>: ' prefix, so the transcript keeps its shape.

Migrate every writeLine/logLine/log helper family (init, vfs, fat,
device-manager, acpi, pci-bus, ps2-bus x3, usb-hid x2, usb-storage,
usb-xhci-bus, virtio-gpu — 104 call sites) to std.log.info, dropping
the hand-written prefixes. A leveled record is a complete line by
contract (raw emissions may still build lines from pieces). Test
fixtures and the display/input services keep raw writes for now — their
ring records are attributed by the kernel regardless.

Harness regexes follow the renamed prefixes (usb-hid-keyboard,
discovery) and path-named restart lines.

system/drivers/ps2-bus/keyboard.zig

@@ -23,11 +23,6 @@ const device = runtime.device;
 const ipc = runtime.ipc;
 const protocol = runtime.input_protocol;
 
-fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
-    var line: [128]u8 = undefined;
-    _ = runtime.system.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
-}
-
 /// Look up the ps2-bus service, retrying while the bus (which spawned us before
 /// registering) is still coming up.
 fn lookupBus() ?ipc.Handle {
@@ -75,14 +70,14 @@ pub fn main(init: runtime.process.Init) void {
         _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: no HID argument\n");
         return;
     }
-    writeLine("/system/drivers/ps2-bus/keyboard: starting for hid {s}\n", .{hid});
+    std.log.info("starting for hid {s}", .{hid});
 
     const buffer = runtime.allocator().alloc(device.DeviceDescriptor, 64) catch {
         _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: out of memory\n");
         return;
     };
     if (device.findDeviceDescriptorByHid(buffer, hid) == null) {
-        writeLine("/system/drivers/ps2-bus/keyboard: no device for hid {s}\n", .{hid});
+        std.log.info("no device for hid {s}", .{hid});
         return;
     }
 
@@ -90,7 +85,7 @@ pub fn main(init: runtime.process.Init) void {
     // absent (as today) it defaults to us.
     const layout_name = init.arguments.get(2) orelse "us";
     const layout = xkb.byName(layout_name) orelse xkb.us;
-    writeLine("/system/drivers/ps2-bus/keyboard: layout {s}\n", .{layout.name});
+    std.log.info("layout {s}", .{layout.name});
 
     // Attach to the bus: hand it our endpoint, and it forwards every byte the
     // keyboard sends (it owns the controller; we own the decoding).