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.

library/runtime/root.zig

@@ -14,6 +14,12 @@ pub const main = program.main;
 /// The panic handler for every safety check in the image (runtime.start.panic).
 pub const panic = runtime.panic;
 
+/// std.log for every user binary goes to the tagged kernel log ring (the kernel
+/// stamps the sender; see runtime.log). A program overrides by declaring its
+/// own `pub const std_options`.
+pub const std_options: @import("std").Options =
+    if (@hasDecl(program, "std_options")) program.std_options else runtime.log.default_options;
+
 comptime {
     _ = &runtime.start._start; // pull the runtime entry shim into the image
 }