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.

build.zig

@@ -56,7 +56,7 @@ fn timestamp(b: *std.Build) []const u8 {
 /// are authoritative — the kernel's W^X user-ELF loader requires exact perms.
 ///
 /// The compilation root is not the program's own file but the shared shim
-/// library/runtime/root.zig, which supplies the root declarations (`main`
+/// library/kernel/root.zig, which supplies the root declarations (`main`
 /// re-export, panic handler, `_start` pull) so a program only defines
 /// `pub fn main`. The program's file becomes the `program` module the shim
 /// imports; reach it through `programModule` to add per-binary imports.
@@ -119,7 +119,7 @@ fn addUserBinaryImpl(
     const exe = b.addExecutable(.{
         .name = name,
         .root_module = b.createModule(.{
-            .root_source_file = b.path("library/runtime/root.zig"),
+            .root_source_file = b.path("library/kernel/root.zig"),
             .target = target,
             .optimize = .ReleaseSmall,
             .code_model = .large,
@@ -133,7 +133,7 @@ fn addUserBinaryImpl(
             },
         }),
     });
-    exe.setLinkerScript(b.path("library/runtime/user.ld"));
+    exe.setLinkerScript(b.path("library/kernel/user.ld"));
     exe.entry = .{ .symbol_name = "_start" };
     exe.image_base = 0x7000_0000_0000;
     exe.use_llvm = true;
@@ -388,7 +388,7 @@ pub fn build(b: *std.Build) void {
     // server. It never touches `boot-handoff` — user space has no business with the
     // loader↔kernel handoff.
     const runtime_module = b.addModule("runtime", .{
-        .root_source_file = b.path("library/runtime/runtime.zig"),
+        .root_source_file = b.path("library/kernel/runtime.zig"),
         .imports = &.{
             .{ .name = "abi", .module = abi_module },
             .{ .name = "device-abi", .module = device_abi_module },
@@ -1028,7 +1028,7 @@ pub fn build(b: *std.Build) void {
     // loop above.
     const time_tests = b.addTest(.{
         .root_module = b.createModule(.{
-            .root_source_file = b.path("library/runtime/time.zig"),
+            .root_source_file = b.path("library/kernel/time.zig"),
             .target = target,
             .optimize = optimize,
             .imports = &.{
@@ -1044,7 +1044,7 @@ pub fn build(b: *std.Build) void {
     // system.zig (syscall wrappers), which needs the `abi` module.
     const thread_tests = b.addTest(.{
         .root_module = b.createModule(.{
-            .root_source_file = b.path("library/runtime/thread.zig"),
+            .root_source_file = b.path("library/kernel/thread.zig"),
             .target = target,
             .optimize = optimize,
             .imports = &.{