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>

sbin/linker.ld

@@ -18,22 +18,29 @@ PHDRS {
 }
 
 SECTIONS {
+    /* The `.large` code model (needed for the >4 GiB image base) emits code and
+     * data into .ltext/.lrodata/.ldata/.lbss; fold those into the matching
+     * permission segment alongside the normal names. */
     .text ALIGN(4K) : {
         *(.text .text.*)
+        *(.ltext .ltext.*)
     } :text
 
     .rodata ALIGN(4K) : {
         *(.rodata .rodata.*)
+        *(.lrodata .lrodata.*)
     } :rodata
 
     .data ALIGN(4K) : {
         *(.data .data.*)
+        *(.ldata .ldata.*)
     } :data
 
     /* .bss occupies memory but not file space; the loader zeroes the
      * filesz..memsz gap. */
     .bss ALIGN(4K) : {
         *(.bss .bss.*)
+        *(.lbss .lbss.*)
         *(COMMON)
     } :data