threads(M7): thread-safe allocation (per-aspace mmap arena + locked heap)

Move the mmap/mmio grant-arena cursors off Task into the per-address-space object
(scheduler aspace_refs, exposed via aspaceMmapNextPtr/aspaceDeviceMapNextPtr), so
sibling threads in one address space hand out disjoint grants. systemMmap reserves
a range under a brief lock then maps per page under a short-held lock (not the
whole grant): the big lock runs with interrupts disabled, so pinning it across a
multi-MiB memset+map froze other cores. Guard the runtime heap's rawAlloc/rawFree
with a Thread.Mutex, gated on !single_threaded so ordinary binaries compile it out.

thread-test gains an alloc mode: 4 threads x 500 alloc/fill/verify/free cycles;
any overlap between concurrent allocations is caught by the pattern check.

Also fix the affinity guardrail: its 3-billion-iteration busy-loop had
codegen-dependent wall-time (adding a function to tests.zig swung it ~4s -> ~63s
and timed it out). Reworked to wait on the wall clock instead.

Gate thread-alloc PASS (3x); full guardrail 23/23 green; build + host tests clean.

test/qemu_test.py

@@ -339,6 +339,14 @@ CASES = [
      "timeout": 60,
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+
+    # docs/threading-plan.md M7: thread-safe allocation — N threads hammer the shared heap
+    # (per-aspace mmap arena + locked free list) with no cross-block corruption.
+    {"name": "thread-alloc",
+     "smp":    4,
+     "timeout": 60,
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Process arguments: argv arrives on the SysV entry stack (argv[0] = the spawned
     # name, argv[1..] = the system_spawn argument blob) and echoes back intact.
     {"name": "args",