reorg: move vfs-protocol to library/protocol + direct import

vfs-protocol -> library/protocol/vfs/vfs-protocol.zig. fat, its only consumer,
now imports the module directly (const vfs_protocol = @import("vfs-protocol"))
instead of through runtime.vfs_protocol, and the runtime re-export is deleted.
The runtime's own client (runtime.fs) still imports the module by name.

zig build + test green; vfs, fat-mount pass.

build.zig

@@ -368,7 +368,7 @@ pub fn build(b: *std.Build) void {
     // is the first "protocol module" (see docs/driver-model.md); usb/block will
     // expose theirs the same way.
     const vfs_protocol_module = b.addModule("vfs-protocol", .{
-        .root_source_file = b.path("system/vfs-protocol.zig"),
+        .root_source_file = b.path("library/protocol/vfs/vfs-protocol.zig"),
     });
 
     // The input wire protocol: the input service's public interface, exposed as its own
@@ -572,6 +572,7 @@ pub fn build(b: *std.Build) void {
     // The FAT filesystem server: mounts the block device and serves it into the VFS
     // at /mnt/usb. Its engine (engine.zig / on-disk.zig) is imported relatively.
     const fat_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "fat", "system/services/fat/fat.zig");
+    programModule(fat_exe).addImport("vfs-protocol", vfs_protocol_module);
     // Threaded: the display runs a mouse-listener thread alongside its compositor loop
     // (docs/threading.md, docs/display.md), so it opts into real atomics/TLS.
     const display_exe = addThreadedUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "display", "system/services/display/display.zig");
@@ -957,7 +958,7 @@ pub fn build(b: *std.Build) void {
         "system/drivers/usb-hid/hid-report.zig", // HID boot-report keyboard/mouse decode
         "system/drivers/usb-storage/bulk-only-transport.zig", // CBW/CSW wrapper sizes
         "system/drivers/usb-storage/scsi.zig", // SCSI CDB encodings (big-endian)
-        "system/vfs-protocol.zig", // NodeKind / DirectoryEntry sizes + op values
+        "library/protocol/vfs/vfs-protocol.zig", // NodeKind / DirectoryEntry sizes + op values
         "system/services/fat/on-disk.zig", // FAT on-disk struct sizes + type detection
         "system/services/fat/engine.zig", // FAT read/write over a RAM-backed image
         "system/services/display/compositor.zig", // Rect math + fill/composite/blit-tile