fix: Apply library structure preservation to _merge_arduino_libs_to_component

_create_idf_component was fixed but NOT _merge_arduino_libs_to_component.
The latter was still flattening library files, causing ArduinoJson compilation to fail
with 'src/ArduinoJson.h: No such file or directory'.

Changes:
- Replace flat copy with directory-structure-preserving copy
- Add exclusion logic for non-buildable directories (examples, tests, docs)
- Generate INCLUDE_DIRS from actual directory structure
- Track files by relative path to prevent name collisions

This ensures libraries with src/ layouts (ArduinoJson, etc.) compile correctly.

backend/app/services/espidf_compiler.py

@@ -454,32 +454,42 @@ class ESPIDFCompiler:
                 found_any = True
                 header_to_comp[header] = 'user_libs_all'
 
-                # Copy all source files flat into the merged component directory.
-                # First-writer wins for name conflicts (rare across Arduino libs).
-                for pattern in ('*.h', '*.cpp', '*.c', 'src/*.h', 'src/*.cpp', 'src/*.c'):
-                    glob_root = src_root.parent if pattern.startswith('src/') else src_root
-                    for f in glob_root.glob(pattern):
-                        if not f.is_file():
-                            continue
-                        if f.name not in seen_names:
-                            shutil.copy2(f, comp_dir / f.name)
-                            seen_names.add(f.name)
-                        if f.suffix in ('.cpp', '.c') and f.name not in cpp_files:
-                            cpp_files.append(f.name)
+                # Preserve directory structure while merging libraries.
+                # Skip non-buildable directories like examples, tests, docs.
+                lib_root = src_root.parent if src_root.name == 'src' else src_root
+                has_src_layout = (lib_root / 'src').is_dir()
 
-                # Also handle libraries that use an src/ subdirectory layout.
-                src_sub = (src_root / 'src') if (src_root / 'src').is_dir() else None
-                if src_sub is None and (src_root.parent / 'src').is_dir():
-                    src_sub = src_root.parent / 'src'
-                if src_sub:
-                    for f in src_sub.glob('**/*'):
-                        if not f.is_file() or f.suffix not in ('.h', '.cpp', '.c'):
-                            continue
-                        if f.name not in seen_names:
-                            shutil.copy2(f, comp_dir / f.name)
-                            seen_names.add(f.name)
-                        if f.suffix in ('.cpp', '.c') and f.name not in cpp_files:
-                            cpp_files.append(f.name)
+                excluded_dirs = {
+                    '.git', '.github', '.vscode', '__pycache__',
+                    'docs', 'doc', 'example', 'examples', 'test', 'tests',
+                    'extras', 'ci', 'fuzz', 'fuzzing', 'benchmark', 'benchmarks',
+                }
+
+                def _should_include(rel_path: Path) -> bool:
+                    parts = rel_path.parts
+                    if any(part.lower() in excluded_dirs for part in parts[:-1]):
+                        return False
+                    if rel_path.suffix not in ('.h', '.hpp', '.c', '.cpp'):
+                        return False
+                    if has_src_layout:
+                        return parts[0] == 'src' or len(parts) == 1
+                    return len(parts) == 1 or parts[0].lower() == 'utility'
+
+                for f in lib_root.rglob('*'):
+                    if not f.is_file():
+                        continue
+                    rel_path = f.relative_to(lib_root)
+                    if not _should_include(rel_path):
+                        continue
+                    # Track file by its relative path to preserve structure
+                    file_key = str(rel_path).replace('\\', '/')
+                    if file_key not in seen_names:
+                        dest = comp_dir / rel_path
+                        dest.parent.mkdir(parents=True, exist_ok=True)
+                        shutil.copy2(f, dest)
+                        seen_names.add(file_key)
+                    if f.suffix in ('.cpp', '.c') and file_key not in cpp_files:
+                        cpp_files.append(file_key)
 
                 # Scan newly copied headers for transitive includes.
                 for lib_file in comp_dir.glob('*.h'):
@@ -497,13 +507,22 @@ class ESPIDFCompiler:
             return [], {}
 
         srcs_line = 'SRCS ' + ' '.join(f'"{f}"' for f in sorted(cpp_files)) if cpp_files else ''
+
+        # Generate INCLUDE_DIRS from the directory structure of copied files
+        include_dirs: set[str] = {'.'}
+        for file_key in seen_names:
+            parent = str(Path(file_key).parent)
+            if parent and parent != '.':
+                include_dirs.add(parent)
+
+        include_dirs_line = 'INCLUDE_DIRS ' + ' '.join(f'"{d}"' for d in sorted(include_dirs))
+
         cmake_content = (
             '# Auto-generated by Velxio — all user libraries merged into one component.\n'
-            '# Single flat directory: every header sees every other header without\n'
-            '# cross-component REQUIRES propagation.\n'
+            '# Directory structure preserved for libraries like ArduinoJson with src/ layout.\n'
             'idf_component_register(\n'
             f'    {srcs_line}\n'
-            '    INCLUDE_DIRS "."\n'
+            f'    {include_dirs_line}\n'
             f'    REQUIRES {arduino_comp_name}\n'
             ')\n'
         )