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## 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) |
|---|---|
|  |  |
## 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)