M4: /sbin/init as PID 1 — a heartbeat process

init is now a real scheduled ring-3 process: it prints a heartbeat and
sleeps, forever. The kernel spawns it at boot via spawnProcess (its own
address space, preemption on) and the boot context drops to idle — the
system's steady state is "kernel idle, init alive in ring 3", beating
~1 Hz. Adds the sleep(ms) syscall. The init/process tests are reshaped
around the heartbeat (repeated syscalls, still-alive, two concurrent
processes on distinct address spaces). Pins init to LLD and folds the
.large code model's .ltext/.lrodata/.ldata into the linker script so its
code segment is R+X. Removes the now-dead borrowed-thread ELF loader
(runInitElf); spawnProcess is the one path. Docs updated. Suite 28/28.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build.zig

@@ -173,6 +173,11 @@ pub fn build(b: *std.Build) void {
     init_exe.setLinkerScript(b.path("sbin/linker.ld"));
     init_exe.entry = .{ .symbol_name = "_start" };
     init_exe.image_base = 0x7000_0000_0000;
+    // The self-hosted linker ignores the script's PHDRS (segment permissions),
+    // which the kernel's W^X user-ELF loader requires (code must be R+X). Pin to
+    // LLVM + LLD so the script is authoritative.
+    init_exe.use_llvm = true;
+    init_exe.use_lld = true;
     b.installArtifact(init_exe);
 
     // Boot methods live in src/boot/, one per way of getting the kernel running.