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/services/fat/fat.zig

@@ -16,11 +16,6 @@ const on_disk = @import("on-disk.zig");
 const protocol = runtime.vfs_protocol;
 const dma = runtime.dma;
 
-fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
-    var line: [96]u8 = undefined;
-    _ = runtime.system.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
-}
-
 const mount_point = "/mnt/usb";
 
 // The engine's BlockDevice, backed by the `.block` driver plus a DMA bounce
@@ -104,14 +99,14 @@ fn initialise(endpoint: runtime.ipc.Handle) bool {
         _ = runtime.system.write("/system/services/fat: not a FAT filesystem\n");
         return false;
     };
-    writeLine("/system/services/fat: mounted FAT ({s}, {d} clusters, partition lba {d})\n", .{ @tagName(filesystem.geometry.fat_type), filesystem.geometry.cluster_count, filesystem.base_lba });
+    std.log.info("mounted FAT ({s}, {d} clusters, partition lba {d})", .{ @tagName(filesystem.geometry.fat_type), filesystem.geometry.cluster_count, filesystem.base_lba });
 
     // Mount ourselves into the VFS namespace at /mnt/usb (retry while the VFS
     // comes up). From here the VFS routes /mnt/usb/... to this server.
     var tries: u32 = 0;
     while (tries < 100) : (tries += 1) {
         if (runtime.fs.mount(mount_point, endpoint)) {
-            writeLine("/system/services/fat: mounted {s}\n", .{mount_point});
+            std.log.info("mounted {s}", .{mount_point});
             return true;
         }
         runtime.system.sleep(50);