M5: rename usermode->process, add yield + mmap/munmap syscalls

Start the user-space driver track (VFS + IPC + heap). This lays the
process/syscall foundation the runtime heap will grow on.

- Rename usermode.zig -> process.zig; drop the retired hello/ping blob and
  its `user` test (subsumed by the real /sbin/init exerciser). Keep the
  isolation-proof pf blob and the user-pf test.
- Add danos.Syscall as the single source of truth for syscall numbers, shared
  by the kernel dispatcher and (later) the user runtime lib. Dispatch on the
  enum. New calls: 1=yield, 4=mmap, 5=munmap. Widen debug_write's bounds
  check to the whole user low half so heap buffers are writable.
- mmap grants zeroed RW+NX pages from a per-process bump arena
  (Task.heap_next, PML4[224] above image+stack); munmap frees the frames.
  Add paging.translateIn / unmapInto (+ arch.translate / unmapUserPageInto)
  as the primitives munmap and future cross-AS copies need.
- New `usermem` test: grant three pages into a fresh AS, translate them,
  release via the munmap path, tear down, and assert no frames leak.
  Suite 28/28 (user -> usermem).

sbin/init.zig

@@ -1,7 +1,7 @@
 //! /sbin/init — the first user-space program, PID 1. Built as its own
 //! freestanding binary (see build.zig), shipped on the boot volume at sbin/init,
 //! loaded by the bootloader, and started in ring 3 as a scheduled process by the
-//! kernel (src/kernel/usermode.zig). It talks to the kernel only through the
+//! kernel (src/kernel/process.zig). It talks to the kernel only through the
 //! `syscall` instruction.
 //!
 //! Today it's a heartbeat: it prints a line and sleeps, forever — enough to show
@@ -11,7 +11,7 @@
 
 const std = @import("std");
 
-// Syscall numbers (see src/kernel/usermode.zig):
+// Syscall numbers (see src/kernel/process.zig):
 const sys_exit = 0;
 const sys_write = 2;
 const sys_sleep = 3;