feat(core): register IPC buffer for extapp remote

### core/embed/projects/firmware/src/coreapp_ipc.h
@@ -0,0 +1,36 @@
+/*
+ * This file is part of the Trezor project, https://trezor.io/
+ *
+ * Copyright (c) SatoshiLabs
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <sys/systask.h>
+
+/**
+ * Task the coreapp exchanges IPC messages with.
+ *
+ * Task IDs are zero-based indices (see `systask_id_t`): 0 is the kernel, 1 the
+ * coreapp itself, 2 the loaded user app. Only meaningful with `USE_APP_LOADING`
+ * enabled, which is also what raises `SYSTASK_MAX_TASKS` to 3.
+ *
+ * TODO: replace with a task id obtained from the applet/app-arena layer once
+ * one is exposed, rather than assuming the single extapp slot.
+ */
+#define IPC_REMOTE_EXTAPP ((systask_id_t)2)
+// Size of the IPC receive buffer registered by the coreapp
+#define IPC_COREAPP_BUFFER_SIZE (32 * 1024)

### core/embed/projects/firmware/src/stm32/main.c
@@ -37,16 +37,16 @@
 #include <io/notify.h>
 #include <io/rsod.h>
 #include <sec/boot_image.h>
+#include <sys/bootutils.h>
 #include <sys/linker_utils.h>
 #include <sys/logging.h>
 #include <sys/systask.h>
 #include <sys/system.h>
+
 #include "rust_ui_common.h"
 
 #include <blake2s.h>
 
-#include "sys/bootutils.h"
-
 #ifdef USE_SECP256K1_ZKP
 #include "zkp_context.h"
 #endif
@@ -55,6 +55,12 @@
 #include <io/ble.h>
 #endif
 
+#ifdef USE_IPC
+#include <sys/ipc.h>
+
+#include "../coreapp_ipc.h"
+#endif
+
 #ifdef USE_NRF
 #include <io/nrf.h>
 
@@ -66,13 +72,24 @@ extern const void nrf_app_size;
 
 LOG_DECLARE(coreapp_main)
 
+#if USE_IPC
+uint32_t ipc_buffer[IPC_COREAPP_BUFFER_SIZE / sizeof(uint32_t)];
+#endif
+
 int main_func(uint32_t cmd, void *arg) {
   if (cmd == 1) {
     systask_postmortem_t *info = (systask_postmortem_t *)arg;
     rsod_gui(info);
     system_exit(0);
   }
 
+#if USE_IPC
+  // Registered once for the coreapp's lifetime: re-registering while an
+  // extapp is running would reset the queue under it, so this is not
+  // something extapp orchestration (e.g. run.py) is allowed to repeat.
+  ipc_register(IPC_REMOTE_EXTAPP, ipc_buffer, sizeof(ipc_buffer));
+#endif
+
   bool fading = DISPLAY_JUMP_BEHAVIOR == DISPLAY_RESET_CONTENT;
 
   bool update_required = false;

### core/embed/projects/firmware/src/unix/main.c
@@ -67,6 +67,12 @@
 #include "shared/runtime/pyexec.h"
 #include "stack_size.h"
 
+#ifdef USE_IPC
+#include <sys/ipc.h>
+
+#include "../coreapp_ipc.h"
+#endif
+
 // Command line options, with their defaults
 bool mp_compile_only = false;
 static uint emit_opt = MP_EMIT_OPT_NONE;
@@ -787,7 +793,17 @@ MP_NOINLINE int main_(int argc, char **argv) {
   return ret & 0xff;
 }
 
+#if USE_IPC
+uint32_t ipc_buffer[IPC_COREAPP_BUFFER_SIZE / sizeof(uint32_t)];
+#endif
+
 int coreapp_emu(int argc, char **argv) {
+#if USE_IPC
+  // Registered once for the coreapp's lifetime; see the stm32 main.c for why
+  // this isn't repeated per extapp launch.
+  ipc_register(IPC_REMOTE_EXTAPP, ipc_buffer, sizeof(ipc_buffer));
+#endif
+
 #if MICROPY_PY_THREAD
   mp_thread_init();
 #endif