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).

test/qemu_test.py

@@ -150,9 +150,9 @@ CASES = [
      "expect": r"page fault \(vector 14\)",
      "fail":   r"NX not enforced"},
     {"name": "fault-null", "expect": r"page fault \(vector 14\)"},
-    # Ring 3: a user program runs at CPL 3, makes int 0x80 syscalls, survives
-    # timer interrupts, and exits back into the kernel.
-    {"name": "user",
+    # Memory grants: the mmap/munmap path hands out user pages into a process's
+    # arena, translate resolves them, munmap frees them, and no frames leak.
+    {"name": "usermem",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Isolation: a ring-3 read of a kernel-only page must #PF with error code