kernel: v4 waits for a key in arch_halt(), not a spin (#151)

An idle StarshipOS kept a CPU core 100% busy: sk_v4_readline(), with
nothing typed, and sk_v4_identity_wait(), headless until Zuse, waited in
arch_relax(). Both now halt until the next interrupt (hlt; wfi on aarch64
and riscv64): the heartbeat's timer, ISR-driven on all three, wakes them at
100 Hz, so a key is still seen within a tick.

Idle at the prompt, QEMU's CPU: 100% -> 1% of a core on amd64, 1% on
aarch64 and riscv64. Acceptance passes: amd64 logs/20261108-005539, aarch64
-005753, riscv64 -010136 (POST 538/538, one dict_hash, 7 ok), the stick
loaded high, test-32 (39 programs, 0 failures). FABRIC-3 XLVIII.5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

kernel/src/v4/sk_v4.c

@@ -113,7 +113,11 @@ static int sk_v4_readline(void)
             } else {
                 heartbeat_service();
             }
-            arch_relax();
+            /* Nothing typed: wait for the next interrupt, not in a spin (#151 -- the spin kept a
+             * CPU core 100% busy at the prompt, and the demo's host with it). The heartbeat's
+             * timer wakes it at 100 Hz, so a key is seen within a tick, as the blink and the
+             * identity beat are. */
+            arch_halt();
             continue;
         }
         if (c == '\r' || c == '\n') {

kernel/src/v4/sk_v4_identity.c

@@ -331,7 +331,7 @@ void sk_v4_identity_wait(void)
     while (!g_zuse_session) {
         sk_v4_discard_input();
         sk_v4_identity_beat();
-        arch_relax();
+        arch_halt();                 /* headless, until Zuse: wait for the next tick, not a spin (#151) */
     }
     sk_v4_discard_input();
 }