Async and base infrastructure for cross-process shared memory. (#23688)

This is the first half of the remote HAL transport stack, split out to
land independently. Everything here is in `iree/async/`, `iree/base/`,
and `iree/testing/` — no net or HAL changes yet.

### Shared memory primitives

Adds `iree_shm_*` for cross-platform shared memory create/open/close/map
with handle passing support (memfd on Linux, shm_open on macOS,
CreateFileMappingW on Windows). Includes memory sealing via
`iree_shm_seal()` for making regions immutable after population — uses
kernel-level F_SEAL_* on Linux, VirtualProtect defense-in-depth on
Windows, unavailable on macOS.

On top of that, a lock-free SPSC queue (`iree_spsc_queue_*`) designed to
operate on caller-provided memory so it can live in a shared memory
region. Monotonically increasing 64-bit positions with acquire-release
ordering, cache-line isolation between producer and consumer, skip
markers for wrap-free reads. This is the data plane for the SHM carrier.

### Cross-process notification

Extends the notification system to support shared-memory epochs and
cross-process wake across all three backends. The core change is an
`epoch_ptr` indirection so notifications can point at either the inline
epoch (local, zero behavioral change) or a caller-provided epoch in
shared memory.

Per-platform wake mechanisms:
- io_uring: shared futex (FUTEX mode) or caller-provided eventfd (EVENT
mode)
- POSIX/Linux: shared futex + caller eventfd for poll loop wake
- POSIX/macOS: poll() on wake fd (no futex, no cross-process condvar)
- IOCP: RegisterWaitForSingleObject bridges caller Event to IOCP;
WakeByAddress is per-process (virtual-address keyed), so cross-process
wake uses the SetEvent path

### Shared buffer pool

Extends `iree_async_buffer_pool_t` with `create_shared`/`open_shared`
for cross-process zero-copy. The atomic freelist (64-bit CAS,
position-independent indices) lives directly in shared memory so both
processes can independently acquire and release buffers. Header magic is
written last as a commit step so openers never see a valid header with
uninitialized freelist state.

### Proactor performance improvements

- **IOCP event waits**: Replace polling-based event wait with
`NtAssociateWaitCompletionPacket` for zero-overhead kernel-level
event-to-IOCP association.
- **IOCP carrier freelist**: Eliminates per-I/O heap allocation in
steady state by recycling carrier structs through an atomic slist
freelist. Pool grows on demand, drained at destroy.
- **IOCP active_carriers**: Doubly-linked list for O(1) carrier removal
(was O(n) list walk on every event wait completion).
- **Timer insertion fast-path**: Tail comparison before list walk turns
the common case (monotonically-increasing deadlines) from O(n) to O(1).
- **Inline progress callbacks**: Proactors can register progress
callbacks that run inline during the event loop, enabling adaptive
polling strategies without full wakeup overhead.

### Bug fixes

- **Send data lifetime in io_uring**: Data referenced by send SQEs
submitted from recv callbacks could be a use-after-return if the
submitting function's stack frame unwound before the kernel read the
data. Fixed by copying send data inline when immediate submission is
possible. New `data_lifetime_test` in the socket CTS.
- **Axis failure propagation**: When an axis operation failed, the
failure status was silently dropped instead of propagating to semaphores
waiting on that axis value.

### Testing infrastructure

Adds a coordinated multi-process test harness (`iree/testing/`). A
single test binary re-executes itself in different roles, with the
launcher orchestrating spawn order, readiness synchronization, and
result collection. Supports Linux, macOS, and Windows. First consumer is
the SHM carrier cross-process tests on the remote-hal branch.

Also adds `iree_async_primitive_dup()`/`close()` for cross-process
handle transfer — the async primitive layer's equivalents of
dup()/close(), DuplicateHandle/CloseHandle, mach_port_mod_refs.

Author: benvanik

runtime/src/iree/async/BUILD.bazel

@@ -44,6 +44,7 @@ iree_runtime_cc_library(
         "frontier_tracker.c",
         "notification.c",
         "operation.c",
+        "primitive.c",
         "proactor.c",
         "region.c",
         "semaphore.c",
@@ -134,6 +135,17 @@ iree_runtime_cc_library(
 # Tests
 #===------------------------------------------------------------------------===#
 
+iree_runtime_cc_test(
+    name = "primitive_test",
+    srcs = ["primitive_test.cc"],
+    deps = [
+        ":async",
+        "//runtime/src/iree/base",
+        "//runtime/src/iree/testing:gtest",
+        "//runtime/src/iree/testing:gtest_main",
+    ],
+)
+
 iree_runtime_cc_test(
     name = "address_test",
     srcs = ["address_test.cc"],

runtime/src/iree/async/CMakeLists.txt

@@ -61,6 +61,7 @@ iree_cc_library(
     "frontier_tracker.c"
     "notification.c"
     "operation.c"
+    "primitive.c"
     "proactor.c"
     "region.c"
     "semaphore.c"
@@ -116,6 +117,18 @@ iree_cc_library(
   PUBLIC
 )
 
+iree_cc_test(
+  NAME
+    primitive_test
+  SRCS
+    "primitive_test.cc"
+  DEPS
+    ::async
+    iree::base
+    iree::testing::gtest
+    iree::testing::gtest_main
+)
+
 iree_cc_test(
   NAME
     address_test