feat: Add File header G-code option and print_time_sec/filament_length_m placeholders (#12186)

## Problem

Some Creality printers (e.g. Ender-3 V3 SE) rely on metadata comments in the first lines of G-code files to display print info on the screen (estimated time, filament usage, layer height). This follows a format originally used by Cura/Creality Print:
```
;FLAVOR:Marlin
;TIME:3708.97
;Filament used:6.21m
;Layer height:0.2
```

OrcaSlicer currently has no way to write content before the `HEADER_BLOCK` in the G-code output, and does not expose `print_time_sec` or `filament_length_m` as usable placeholders. As a result, these printers show zeroed-out print info on screen.

## Changes

### 1. New placeholders (post-processing)
Added two new G-code placeholders resolved during post-processing:
- `{print_time_sec}` — total estimated print time in seconds (double)
- `{filament_length_m}` — total filament length in meters (double)

These values are only available after G-code generation, so they use inline marker replacement (`@PRINT_TIME_SEC@`, `@FILAMENT_LENGTH_M@`) that `GCodeProcessor::run_post_process()` substitutes with actual computed values — the same pattern used by existing M73 and layer count placeholders.

### 2. New "File header G-code" option (`machine_top_gcode`)
Added a new G-code field in **Printer Settings > Machine G-code** that writes content at the very top of the output file, before `HEADER_BLOCK_START`. This allows users to add firmware-specific metadata that must appear in the first lines.

### 3. Creality Ender-3 V3 SE profile update
Pre-configured the Ender-3 V3 SE profiles (all 4 nozzle variants) with a default `machine_top_gcode` value matching the Cura-compatible header format, so print info displays correctly out of the box.

# Screenshots

| Before (current OrcaSlicer) | After (this PR) |
|---|---|
| ![IMG_5285](https://github.com/user-attachments/assets/fdd2cc96-6593-4108-83f4-e272d5547467) | ![IMG_5286](https://github.com/user-attachments/assets/6cb4091a-4560-4596-93d9-49f2ff8f0a2b) |

## Tests

- Sliced test model with Ender-3 V3 SE profile
- Verified G-code output contains correct values in first lines:
  - `;TIME:` with actual print time in seconds
  - `;Filament used:` with filament length in meters
  - `;Layer height:` with correct layer height
- Verified `{print_time_sec}` and `{filament_length_m}` work in both Machine Start G-code and File header G-code fields
- Verified empty `machine_top_gcode` produces no extra output
- Built and tested on macOS (arm64)

src/libslic3r/GCode.cpp

@@ -2313,6 +2313,19 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
     if (!print.config().small_area_infill_flow_compensation_model.empty())
         m_small_area_infill_flow_compensator = make_unique<SmallAreaInfillFlowCompensator>(print.config());
     
+    // Process file_start_gcode - written at the very top of the file, before any header
+    {
+        std::string top_gcode_template = print.config().file_start_gcode.value;
+        if (!top_gcode_template.empty()) {
+            DynamicConfig top_config;
+            top_config.set_key_value("print_time_sec", new ConfigOptionString(GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Print_Time_Sec_Placeholder)));
+            top_config.set_key_value("used_filament_length", new ConfigOptionString(GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Used_Filament_Length_Placeholder)));
+            std::string top_gcode = print.placeholder_parser().process(top_gcode_template, 0, &top_config);
+            if (!top_gcode.empty())
+                file.writeln(top_gcode);
+        }
+    }
+
     // Orca: Don't output Header block if BTT thumbnail is identified in the list
     // Get the thumbnails value as a string
     std::string thumbnails_value = print.config().option<ConfigOptionString>("thumbnails")->value;
@@ -2854,6 +2867,8 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
         this->placeholder_parser().set("hold_chamber_temp_for_flat_print", new ConfigOptionBool(hold_chamber_temp_for_flat_print));
     }
 
+    this->placeholder_parser().set("print_time_sec", new ConfigOptionString(GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Print_Time_Sec_Placeholder)));
+    this->placeholder_parser().set("used_filament_length", new ConfigOptionString(GCodeProcessor::reserved_tag(GCodeProcessor::ETags::Used_Filament_Length_Placeholder)));
 
     std::string machine_start_gcode = this->placeholder_parser_process("machine_start_gcode", print.config().machine_start_gcode.value, initial_extruder_id);
     if (print.config().gcode_flavor != gcfKlipper) {