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/GCodeProcessor.cpp

@@ -73,7 +73,9 @@ const std::vector<std::string> GCodeProcessor::Reserved_Tags = {
     "_DURING_PRINT_EXHAUST_FAN",
     " WIPE_TOWER_START",
     " WIPE_TOWER_END",
-    " PA_CHANGE:"
+    " PA_CHANGE:",
+    "@PRINT_TIME_SEC@",
+    "@USED_FILAMENT_LENGTH@"
 };
 
 const std::vector<std::string> GCodeProcessor::Reserved_Tags_compatible = {
@@ -94,7 +96,9 @@ const std::vector<std::string> GCodeProcessor::Reserved_Tags_compatible = {
     "_DURING_PRINT_EXHAUST_FAN",
     " WIPE_TOWER_START",
     " WIPE_TOWER_END",
-    " PA_CHANGE:"
+    " PA_CHANGE:",
+    "@PRINT_TIME_SEC@",
+    "@USED_FILAMENT_LENGTH@"
 };
 
 
@@ -1101,6 +1105,42 @@ void GCodeProcessor::run_post_process()
         return ret;
     };
 
+    // Process inline placeholders (print_time_sec and used_filament_length)
+    auto process_inline_placeholders = [&](std::string& gcode_line) {
+        bool processed = false;
+
+        const std::string& print_time_placeholder = reserved_tag(ETags::Print_Time_Sec_Placeholder);
+        const std::string& used_filament_placeholder = reserved_tag(ETags::Used_Filament_Length_Placeholder);
+
+        // Replace print_time_sec
+        size_t pos = gcode_line.find(print_time_placeholder);
+        while (pos != std::string::npos) {
+            double print_time_sec = m_time_processor.machines[static_cast<size_t>(PrintEstimatedStatistics::ETimeMode::Normal)].time;
+            char buf[64];
+            sprintf(buf, "%.2f", print_time_sec);
+            gcode_line.replace(pos, print_time_placeholder.length(), buf);
+            processed = true;
+            pos = gcode_line.find(print_time_placeholder, pos + strlen(buf));
+        }
+
+        // Replace used_filament_length
+        pos = gcode_line.find(used_filament_placeholder);
+        while (pos != std::string::npos) {
+            double total_filament_mm = 0.0;
+            for (const auto& mm : filament_mm) {
+                total_filament_mm += mm;
+            }
+            double used_filament_length = total_filament_mm / 1000.0; // Convert mm to m
+            char buf[64];
+            sprintf(buf, "%.2f", used_filament_length);
+            gcode_line.replace(pos, used_filament_placeholder.length(), buf);
+            processed = true;
+            pos = gcode_line.find(used_filament_placeholder, pos + strlen(buf));
+        }
+
+        return processed;
+    };
+
     // check for temporary lines
     auto is_temporary_decoration = [](const std::string_view gcode_line) {
         // remove trailing '\n'
@@ -1317,6 +1357,8 @@ void GCodeProcessor::run_post_process()
                         gcode_line.clear();
                     if (!processed)
                         processed = process_used_filament(gcode_line);
+                    if (!gcode_line.empty())
+                        process_inline_placeholders(gcode_line);
                     if (!processed && !is_temporary_decoration(gcode_line)) {
                         if (GCodeReader::GCodeLine::cmd_is(gcode_line, "G0") || GCodeReader::GCodeLine::cmd_is(gcode_line, "G1")) {
                             export_line.append_line(gcode_line);