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.

test/qemu_test.py

@@ -116,6 +116,11 @@ CASES = [
     {"name": "ipc",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Synchronous IPC: a client and server ping-pong 100 calls through an
+    # endpoint (rendezvous, reply routing, cross-address-space copy).
+    {"name": "ipc-call",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Parallelism: needs more than one core, so this case boots with -smp 4.
     {"name": "smp",
      "smp":    4,