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).

src/root.zig

@@ -58,6 +58,26 @@ pub const page_size = 4096;
 pub const physmap_base: u64 = 0xFFFF_8800_0000_0000;
 pub const kernel_virt_base: u64 = 0xFFFF_FFFF_8000_0000;
 
+/// The kernel syscall numbers — the single source of truth shared by the kernel
+/// dispatcher (src/kernel/process.zig) and the user runtime library, so the two
+/// can never drift. The set is deliberately microkernel-minimal: file/device I/O
+/// is not here — it lives in user-space servers reached through the IPC calls.
+/// The table grows one milestone at a time; see docs/syscall.md.
+pub const Syscall = enum(u64) {
+    exit = 0, // exit(code): end the calling process
+    yield = 1, // yield(): give up the rest of this quantum
+    debug_write = 2, // debug_write(ptr, len): raw bytes to the kernel log (bring-up only)
+    sleep = 3, // sleep(ms): block the caller for ms milliseconds
+    mmap = 4, // mmap(len, prot) -> base: grant zeroed, page-aligned user pages
+    munmap = 5, // munmap(base, len): release pages from a prior mmap
+    _,
+};
+
+/// Protection flags for `mmap` (matching the usual C bit values).
+pub const prot_read: u64 = 1;
+pub const prot_write: u64 = 2;
+pub const prot_exec: u64 = 4;
+
 /// Physical address -> its virtual address in the physmap. The single way the
 /// kernel dereferences a physical address once paging is up.
 ///