Implement app streams: stdio for apps (#640)

Implement file IO for applications to facilitate text input/output capturing.

Apps can now launch apps and:
- Read their stdio output
- Write to their stdio input

Modules/app-module/include/app/file.h

@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: Apache-2.0
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+#include <sys/types.h>
+
+#include <tactility/error.h>
+#include <tactility/freertos/freertos.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** Which readiness condition to block for in AppFileOps::await(). */
+typedef enum {
+    APP_FILE_WAIT_READABLE,
+    APP_FILE_WAIT_WRITABLE,
+} AppFileWait;
+
+/** Bits returned by AppFileOps::poll(). */
+#define APP_FILE_READABLE (1u << 0)
+#define APP_FILE_WRITABLE (1u << 1)
+
+/**
+ * Operations table for one kind of file-like object (stream, file, device, ...). Every function
+ * takes the type-erased `object` a concrete AppFile instance was constructed with.
+ */
+struct AppFileOps {
+    ssize_t (*read)(void* object, void* buffer, size_t size);
+    ssize_t (*write)(void* object, const void* buffer, size_t size);
+    error_t (*close)(void* object);
+    error_t (*await)(void* object, AppFileWait wait, TickType_t timeout);
+    /** @return bitmask of APP_FILE_READABLE / APP_FILE_WRITABLE. */
+    uint32_t (*poll)(void* object);
+    /** Optional (may be NULL). Called by app_fd_table_get_and_retain() atomically with the fd
+     * lookup, before handing `object` to a caller about to dispatch into it, so a concurrent
+     * teardown of `object` can wait for that dispatch to finish instead of racing it. The caller
+     * calls release() once done, regardless of what the dispatched call returned. */
+    void (*retain)(void* object);
+    void (*release)(void* object);
+};
+
+/** A file-descriptor-table entry: an operations table paired with the object it operates on. */
+struct AppFile {
+    const struct AppFileOps* ops;
+    void* object;
+};
+
+#ifdef __cplusplus
+}
+#endif