First layer: AFC fault detection less sensitive

A large, slow first layer tripped AFC's buffer fault detection ("AFC NOT
FEEDING"). _FIRST_LAYER_FAULT_DETECTION ON=1 at the end of PRINT_START
sets first_layer_sensitivity (4: pause after 70 mm instead of 30 mm);
ON=0 restores the [AFC_buffer] config value. The slicer calls ON=0 at
the start of layer 2:
  {if layer_num == 1}_FIRST_LAYER_FAULT_DETECTION ON=0{endif}
_PRINT_CLEANUP calls it too, for prints that end in the first layer.
ON=0 only restores what ON=1 changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -21,7 +21,7 @@ everything below, grouped by topic. Comment out what you do not use there.
 | `blobifier.cfg` | `BLOBIFIER`, `BLOBIFIER_CLEAN`, `BLOBIFIER_PARK`, `BLOBIFIER_SHAKE_BUCKET`, ...: purge bucket and nozzle brush for AFC (Turtleblobifier, heavily modified) | Blobifier hardware, AFC |
 | `blobifier_hw.cfg` | servo and bucket switch pins | — |
 | **print/** | | |
-| `print_start_end.cfg` | `PRINT_WARMUP`, `PRINT_START`, `PRINT_END` | Beacon, AFC, Blobifier, `HEAT_SOAK`, iHeater, bd_pressure PA macros (bdwidth is no longer enabled while printing) |
+| `print_start_end.cfg` | `PRINT_WARMUP`, `PRINT_START`, `PRINT_END`; `_FIRST_LAYER_FAULT_DETECTION`: AFC buffer fault detection less sensitive during the first layer (needs one line in the slicer's layer change G-code, see the macro) | Beacon, AFC, Blobifier, `HEAT_SOAK`, iHeater, bd_pressure PA macros (bdwidth is no longer enabled while printing) |
 | `pause_resume.cfg` | `PAUSE`, `RESUME`, `CANCEL_PRINT`, `M600` (RESUME also handles a running `HEAT_SOAK`, and after a PAUSE purges 40 mm with `BLOBIFIER_CHECKED` first: a failed check keeps the print paused, `RESUME PURGE=0` skips the purge; CANCEL_PRINT runs the same cleanup as `PRINT_END`) | `heating/heat_soak.cfg`, `leds/`, `blobifier/`, `afc/purge_check.cfg` |
 | `filament.cfg` | `LOAD_FILAMENT`, `UNLOAD_FILAMENT` | — |
 | `homing.cfg` | `HOME`: X/Y, then Z always with Beacon contact (also when already homed); `_Z_CHECK`: stops the print when Klipper's Z and the Beacon reading differ (run in `PRINT_START` after the soak) | Beacon |