refactor(fl): replace int return codes with fl_error_t enum

Extract fl_error_t into standalone fl_error.h to avoid header
coupling between fl.h and fl_stream.h.

- FL_OK(0), FL_ERR_ARGS(-1), FL_ERR_CRC(-2), FL_ERR_PARSE(-3),
  FL_ERR_UNKNOWN(-4)
- cmd_handler_t, verify_args_crc, verify_patch_crc, fl_exec_cmd,
  fl_stream_exec_line all return fl_error_t
- fl_port_nuttx.c uses fl_error_t for fl_exec_cmd return value
- All test assertions updated: int result -> fl_error_t result,
  0 -> FL_OK, != 0 -> != FL_OK, -1 -> FL_ERR_ARGS
- Single-line if bodies wrapped with braces

Author: FASTSHIFT

App/func_loader/fl.c

@@ -1240,9 +1240,10 @@ static const cmd_entry_t s_cmd_table[] = {
 
 #define CMD_TABLE_SIZE (sizeof(s_cmd_table) / sizeof(s_cmd_table[0]))
 
-int fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv) {
-    if (argc == 0)
-        return -1;
+fl_error_t fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv) {
+    if (argc == 0) {
+        return FL_ERR_ARGS;
+    }
 
     cmd_args_t args = {0};
     args.crc = -1;
@@ -1273,7 +1274,7 @@ int fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv) {
     fl_argparse_init(&ap, opts, NULL, 0);
     if (fl_argparse_parse(&ap, argc, argv) > 0) {
         fl_response(false, "Invalid arguments");
-        return -1;
+        return FL_ERR_PARSE;
     }
 
     if (!args.cmd) {
@@ -1282,7 +1283,7 @@ int fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv) {
             fl_println("  %s", s_cmd_table[i].name);
         }
         fl_response(false, "Missing --cmd");
-        return -1;
+        return FL_ERR_ARGS;
     }
 
     /* Lookup command in dispatch table */
@@ -1293,5 +1294,5 @@ int fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv) {
     }
 
     fl_response(false, "Unknown: %s", args.cmd);
-    return -1;
+    return FL_ERR_UNKNOWN;
 }

App/func_loader/fl.h

@@ -36,6 +36,7 @@ extern "C" {
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include "fl_error.h"
 #include "fl_file.h"
 
 /* Maximum slot count (FPB v1: 6, v2: 8) */
@@ -118,9 +119,9 @@ bool fl_is_inited(fl_context_t* ctx);
 
 /**
  * @brief Execute command from argc/argv
- * @return 0 on success, -1 on error
+ * @return FL_OK on success, negative fl_error_t on error
  */
-int fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv);
+fl_error_t fl_exec_cmd(fl_context_t* ctx, int argc, const char** argv);
 
 #ifdef __cplusplus
 }

App/func_loader/fl_error.h

@@ -0,0 +1,51 @@
+/*
+ * MIT License
+ * Copyright (c) 2026 VIFEX
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+/**
+ * @file   fl_error.h
+ * @brief  Error codes for func_loader
+ */
+
+#ifndef FL_ERROR_H
+#define FL_ERROR_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Error codes for fl_exec_cmd and command handlers
+ */
+typedef enum {
+    FL_OK = 0,           /* Command executed (success or handled error) */
+    FL_ERR_ARGS = -1,    /* Missing or invalid required arguments */
+    FL_ERR_CRC = -2,     /* CRC verification failed */
+    FL_ERR_PARSE = -3,   /* Argument parsing failed */
+    FL_ERR_UNKNOWN = -4, /* Unknown command */
+} fl_error_t;
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* FL_ERROR_H */

App/func_loader/fl_port_nuttx.c

@@ -188,8 +188,8 @@ static int interactive_mode(fl_context_t* ctx, int argc_first, char** argv_first
 
         int argc = parse_line(line, argv, sizeof(argv) / sizeof(argv[0]));
         if (argc > 0) {
-            const int ret = fl_exec_cmd(ctx, argc, argv);
-            if (ret != 0) {
+            const fl_error_t ret = fl_exec_cmd(ctx, argc, argv);
+            if (ret != FL_OK) {
                 printf("Type 'q' to exit\n\n");
             }
         }

App/func_loader/fl_stream.c

@@ -84,13 +84,13 @@ static int parse_line(char* line, const char** argv, int max_argc) {
     return argc;
 }
 
-int fl_stream_exec_line(fl_stream_t* s, char* line) {
+fl_error_t fl_stream_exec_line(fl_stream_t* s, char* line) {
     static const char* argv[FL_MAX_ARGC];
     int argc = parse_line(line, argv, FL_MAX_ARGC);
     if (argc > 0) {
         return fl_exec_cmd(s->ctx, argc, argv);
     }
-    return 0;
+    return FL_OK;
 }
 
 void fl_stream_process(fl_stream_t* s) {

App/func_loader/fl_stream.h

@@ -35,6 +35,7 @@ extern "C" {
 
 #include <stdint.h>
 #include <stddef.h>
+#include "fl_error.h"
 
 /* Serial callbacks */
 typedef int (*fl_serial_read_cb_t)(uint8_t* buf, size_t len);
@@ -71,7 +72,7 @@ void fl_stream_process(fl_stream_t* s);
 /**
  * @brief Parse line and execute
  */
-int fl_stream_exec_line(fl_stream_t* s, char* line);
+fl_error_t fl_stream_exec_line(fl_stream_t* s, char* line);
 
 #ifdef __cplusplus
 }