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
 }

library/kernel/runtime.zig

@@ -1,51 +0,0 @@
-//! runtime.zig — a **compatibility shim** for the runtime split (reorg C1–C5).
-//!
-//! `library/runtime` became `library/kernel`, and the one giant `runtime` module is being
-//! split into directly-importable concern modules (`ipc`, `memory`, `process`, `time`,
-//! `logging`, `file-system`, `service`, `thread`, `start`, plus the device/service clients).
-//! This file re-exports those modules under the old `runtime.*` names so the ~38 consumers
-//! keep compiling until each is migrated to direct imports. Deleted in step C5.
-
-pub const system = @import("system"); // the system.zig compatibility shim
-pub const ipc = @import("ipc");
-pub const memory = @import("memory");
-pub const allocator = memory.allocator;
-pub const process = @import("process");
-pub const service = @import("service");
-pub const Thread = @import("thread").Thread;
-pub const time = @import("time");
-pub const log = @import("logging");
-pub const logging = @import("logging");
-pub const fs = @import("file-system");
-pub const start = @import("start");
-pub const panic = start.panic;
-
-// Device / service clients (now in library/device/driver, library/device/block, library/client).
-pub const device = @import("driver");
-pub const device_manager = @import("driver"); // hello() folded into driver
-pub const input = @import("input");
-pub const block = @import("block");
-pub const display = @import("display");
-
-/// The heap as a namespace (`runtime.heap.allocator()`), plus `runtime.allocator`.
-pub const heap = struct {
-    pub const allocator = memory.allocator;
-};
-
-/// `runtime.dma.*` mapped onto the flat `memory` API (memory groups heap+dma+shared-memory).
-pub const dma = struct {
-    pub const Region = memory.DmaRegion;
-    pub const coherent = memory.dma_coherent;
-    pub const write_combining = memory.dma_write_combining;
-    pub const below_4g = memory.dma_below_4g;
-    pub const alloc = memory.dmaAlloc;
-    pub const free = memory.dmaFree;
-};
-
-/// `runtime.shared_memory.*` mapped onto the flat `memory` API.
-pub const shared_memory = struct {
-    pub const Region = memory.SharedRegion;
-    pub const create = memory.sharedCreate;
-    pub const map = memory.sharedMap;
-    pub const physical = memory.sharedPhysical;
-};

library/kernel/system.zig

@@ -1,66 +0,0 @@
-//! system.zig — a **compatibility shim**, not the real home of anything anymore.
-//!
-//! The runtime's syscall surface used to be dumped here in one file. It has been split by
-//! concern into `time` / `logging` / `process` / `file-system` / `memory`. This re-exports
-//! the old flat `system.*` names from those homes so consumers that still write
-//! `runtime.system.write` (etc.) keep compiling until they migrate to the concern modules.
-//! Deleted once nothing references `runtime.system` (reorg step C5).
-
-const memory = @import("memory");
-const process = @import("process");
-const time = @import("time");
-const logging = @import("logging");
-const file_system = @import("file-system");
-
-// memory
-pub const PROT_READ = memory.PROT_READ;
-pub const PROT_WRITE = memory.PROT_WRITE;
-pub const PROT_EXEC = memory.PROT_EXEC;
-pub const mmap = memory.mmap;
-pub const munmap = memory.munmap;
-pub const mmapFailed = memory.mmapFailed;
-
-// process
-pub const ProcessDescriptor = process.ProcessDescriptor;
-pub const yield = process.yield;
-pub const exit = process.exit;
-pub const spawn = process.spawn;
-pub const spawnWithArguments = process.spawnWithArguments;
-pub const spawnSupervised = process.spawnSupervised;
-pub const processes = process.processes;
-pub const isProcessRunning = process.isProcessRunning;
-pub const kill = process.kill;
-
-// time
-pub const clock = time.clock;
-pub const wallClock = time.wallClock;
-pub const sleep = time.sleepMillis;
-pub const timerOnce = time.timerOnce;
-
-// logging
-pub const write = logging.write;
-pub const writeRecord = logging.writeRecord;
-pub const klogRead = logging.klogRead;
-pub const klogStatus = logging.klogStatus;
-pub const KlogLevel = logging.KlogLevel;
-pub const KlogStatus = logging.KlogStatus;
-pub const KlogRecordHeader = logging.KlogRecordHeader;
-pub const klog_record_header_size = logging.klog_record_header_size;
-pub const klog_record_alignment = logging.klog_record_alignment;
-pub const klog_record_magic = logging.klog_record_magic;
-pub const klog_flag_truncated = logging.klog_flag_truncated;
-pub const klog_maximum_message = logging.klog_maximum_message;
-pub const maximum_process_name = logging.maximum_process_name;
-
-// file-system
-pub const FsRoute = file_system.FsRoute;
-pub const fsResolve = file_system.fsResolve;
-pub const fsNodeRead = file_system.fsNodeRead;
-pub const fsNodeStatus = file_system.fsNodeStatus;
-pub const fsNodeReaddir = file_system.fsNodeReaddir;
-pub const fsMount = file_system.fsMount;
-pub const fsUnmount = file_system.fsUnmount;
-pub const FileAttributes = file_system.FileAttributes;
-pub const DirectoryEntryHeader = file_system.DirectoryEntryHeader;
-pub const file_kind_regular = file_system.file_kind_regular;
-pub const file_kind_directory = file_system.file_kind_directory;