reorg: rename library/runtime -> library/kernel (groundwork)

First step of splitting the runtime dumping ground. Pure directory rename
(git mv library/runtime library/kernel) + the six build.zig path references
repointed. The module is still named "runtime" for now; the next commits split
it into concern modules (ipc, memory, process, time, logging, file-system, ...),
dissolve system.zig, and delete the runtime aggregator.

The library/kernel name follows the kernel32 model: it is the userspace library
that wraps the private kernel ABI, distinct from system/kernel/ (the kernel).

zig build green.

library/kernel/runtime.zig

@@ -0,0 +1,74 @@
+//! danos user-space runtime library — a nascent libc. Every user binary (init,
+//! and later the VFS server + device drivers) imports this as `@import("runtime")`:
+//! system_call wrappers, the C-convention heap, IPC helpers, and the process start
+//! shim. It is compiled into each binary (inheriting its `.large` code model and
+//! freestanding target), so all user programs share one implementation.
+//!
+//! A user binary only defines a `pub fn main() void` or
+//! `pub fn main(init: runtime.process.Init) void` (arguments arrive via `init`).
+//! The panic handler and the `_start` entry pull live in the shared compilation
+//! root, library/runtime/root.zig, which build.zig wires around every program —
+//! nothing to declare per source file.
+
+pub const system = @import("system.zig");
+pub const log = @import("log.zig");
+/// Monotonic time, delays, and deadlines over the kernel clock/sleep/timer syscalls
+/// — an `Instant`/`Duration` front door, no time service (docs/timers.md).
+pub const time = @import("time.zig");
+pub const heap = @import("heap.zig");
+pub const ipc = @import("ipc.zig");
+pub const start = @import("start.zig");
+
+/// Client for talking to the device manager (the hello handshake a supervised
+/// driver owes at startup). See library/runtime/device-manager.zig. The wire
+/// protocol itself is the library/protocol/device-manager module, imported
+/// directly by drivers and services that speak it.
+pub const device_manager = @import("device-manager.zig");
+/// Keyboard-event listening (subscribe/next) and broadcasting (publish), over the input
+/// service. See library/runtime/input.zig and system/services/input/.
+pub const input = @import("input.zig");
+/// POSIX-style file API: open/read/write/lseek/stat/close.
+/// C stdio: fopen/fread/fwrite/fseek/ftell/fclose over unistd.
+/// Device access for drivers: enumerate/claim/mmioMap.
+pub const device = @import("device.zig");
+/// DMA-capable memory for drivers: contiguous, pinned, uncacheable buffers.
+pub const dma = @import("dma.zig");
+
+/// Shared cacheable memory: create a region + capability, pass the capability to another
+/// process (an `ipc_call` send_cap), map the same pages there. See library/runtime/shared-memory.zig
+/// and docs/display-v2.md.
+pub const shared_memory = @import("shared-memory.zig");
+
+// The USB class-driver client moved to its domain home, library/device/usb (module
+// "usb"): it is bus-family logic, not core runtime, and re-exporting it here compiled it
+// into every binary. USB class drivers import it directly with @import("usb").
+
+/// Block-device client: read/write a block device (a USB stick, via
+/// usb-storage). See library/runtime/block.zig.
+pub const block = @import("block.zig");
+
+/// Display-service client: query the mode, and (from D3) create layers, draw, and
+/// present frames. See library/runtime/display.zig and system/services/display/.
+pub const display = @import("display.zig");
+
+/// The danos-native file API (open/read/write/list over the user-space VFS) — the
+/// layer danos programs use directly, and where the operations that later become
+/// `std.os.danos` are staged. See docs/zig-self-hosting.md.
+pub const fs = @import("fs.zig");
+
+/// Re-exported so the root shim (root.zig) can install it as the panic handler.
+pub const panic = start.panic;
+
+/// Process entry types: the `Init` handed to `main`, and its `Arguments`.
+pub const process = @import("process.zig");
+
+/// Threads: `runtime.Thread`, std.Thread-shaped, over the private thread ABI
+/// (docs/threading.md). A binary must be built multi-threaded to spawn.
+pub const Thread = @import("thread.zig").Thread;
+
+/// The service harness: one replyWait loop folding requests, signals, and
+/// notifications into callbacks (docs/process-lifecycle.md).
+pub const service = @import("service.zig");
+
+/// The heap as a `std.mem.Allocator`, for Zig `std` containers in user code.
+pub const allocator = heap.allocator;