M7: synchronous IPC — endpoints, handle table, IPC_Call/ReplyWait

The microkernel message backbone the VFS server and drivers will ride on.

- src/kernel/ipc_sync.zig: Endpoint (sender FIFO threaded via Task.next +
  a recv WaitQueue for servers + a small notification ring). call() (client
  sends, wakes a server, blocks) and replyWait() (server replies to the held
  caller, then receives the next). Reply routing keys on Task.ipc_client —
  synchronous IPC owes one reply at a time. Payloads copy frame-to-frame
  through the physmap (copyAcross on arch.translate, added in M5); an unmapped
  page fails the copy instead of #PF-ing. MSG_MAX 256.
- Bootstrap naming: an integer name registry (danos.ServiceId, vfs=1) with
  create_endpoint / ipc_register / ipc_lookup — any process finds a server
  without threading a handle through spawn.
- notifyFromIsr(): ISR-safe async wake (badge with the high bit set), the
  hook M10's IRQ-as-message needs. Unused/untested until then.
- Task gains handles[16] (opaque *Endpoint, to avoid a sched<->ipc import
  cycle) + ipc_client/send/reply/status fields; sched gains
  blockCurrentLocked/readyLocked; arch gains setSyscallResult2 (rdx badge).
- Syscalls 6..10 wired in process.zig; exit() now drops the caller's endpoint
  refs. lib/ipc.zig: user-side createEndpoint/register/lookup/call
  (server-side replyWait lands with the first server in M9).
- New `ipc-call` test: two kernel tasks ping-pong 100 calls, every reply
  request+1. Suite 29/29.

src/root.zig

@@ -70,6 +70,19 @@ pub const Syscall = enum(u64) {
     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
+    create_endpoint = 6, // create_endpoint() -> handle: a new IPC endpoint
+    ipc_register = 7, // ipc_register(service_id, handle): publish an endpoint by well-known id
+    ipc_lookup = 8, // ipc_lookup(service_id) -> handle: find a published endpoint
+    ipc_call = 9, // ipc_call(h, msg, len, reply, cap) -> reply_len: send + block for reply
+    ipc_reply_wait = 10, // ipc_reply_wait(h, reply, len, recv, cap) -> recv_len (+badge in rdx)
+    _,
+};
+
+/// Well-known IPC service ids for the bootstrap name registry (create_endpoint +
+/// ipc_register/ipc_lookup). Small integers, so no string interning is needed
+/// during bring-up. The VFS server registers under `vfs`; clients look it up.
+pub const ServiceId = enum(u32) {
+    vfs = 1,
     _,
 };