C5: delete the runtime.zig and system.zig compatibility shims

With every consumer migrated (C2), nothing imports `runtime` or `system` any
longer. Removed both shim files and their build.zig module definitions, dropped
the `runtime` entry from `default_imports`, and rewrote the per-binary root shim
(library/kernel/root.zig) to import `start` (_start + panic) and `logging`
(std_options) directly instead of through `runtime`.

addUserBinary/addThreadedUserBinary/addUserBinaryImpl lose their `runtime_module`
parameter; the root module now pulls the two modules it needs out of
`default_imports` via a small findImport helper.

The runtime dumping ground is gone: the userspace library is library/kernel
(concern modules), library/device (driver/pci/usb/block/model/mmio/acpi data),
library/client (display, input service clients), and library/protocol (wire
contracts). Consumers @import concern modules by name.

Verified: zig build, zig build test, and QEMU (smoke, thread-spawn, thread-mutex,
logger, display-native, device-manager, usb-storage, input).

library/kernel/root.zig

@@ -1,25 +1,26 @@
 //! The root module every user binary is compiled through (build.zig,
 //! `addUserBinary`). The program's own file is imported as `program`, and this
 //! shim contributes the declarations Zig resolves from the compilation root —
-//! `main` (dispatched by runtime.start) and the panic handler — and pulls in the
-//! `_start` entry shim. A program therefore only defines `pub fn main`; nothing
-//! else is required in its source file.
+//! `main` (dispatched by start's comptime dispatch) and the panic handler — and
+//! pulls in the `_start` entry shim. A program therefore only defines
+//! `pub fn main`; nothing else is required in its source file.
 
-const runtime = @import("runtime");
+const start = @import("start");
+const logging = @import("logging");
 const program = @import("program");
 
-/// Resolved as `@import("root").main` by runtime.start's comptime dispatch.
+/// Resolved as `@import("root").main` by start's comptime dispatch.
 pub const main = program.main;
 
-/// The panic handler for every safety check in the image (runtime.start.panic).
-pub const panic = runtime.panic;
+/// The panic handler for every safety check in the image (start.panic).
+pub const panic = start.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`.
+/// stamps the sender; see the logging module). 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;
+    if (@hasDecl(program, "std_options")) program.std_options else logging.default_options;
 
 comptime {
-    _ = &runtime.start._start; // pull the runtime entry shim into the image
+    _ = &start._start; // pull the entry shim into the image
 }