Chamber prints: unload a lane that is not the first tool before the soak

With the chamber heated, a filament left in the toolhead path softens in
the PTFE tube (PLA loaded, ASA print). After the first HOME, PRINT_WARMUP
unloads the loaded lane when it is not mapped to the print's first tool
(PRINT_START would swap it anyway). A failed unload stops the print
before the chamber heats.

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

macros/print/print_start_end.cfg

@@ -19,6 +19,7 @@ description: Print start before HEAT_SOAK: home while the bed heats, wait for th
 gcode:
     {% set target_bed = params.BED|int %}
     {% set target_chamber = params.CHAMBER|default("0")|int %}
+    {% set initial_tool = params.TOOL|default("0")|int %}
 
     RESET_FAILURE
     CLEAR_PAUSE
@@ -48,6 +49,9 @@ gcode:
     _NOZZLE_CONTACT_TEMP
     RESPOND MSG="homing"
     HOME
+    {% if target_chamber > 30 %}
+      _UNLOAD_FOR_CHAMBER TOOL={initial_tool}
+    {% endif %}
     G1 X175 Y175 Z5 F1200
 
     _STATUS STATE=heating_bed
@@ -67,6 +71,38 @@ gcode:
       HEAT_SOAK HEATER='heater_bed' TARGET={target_bed} SOAKER='temperature_sensor bed_core' SOAK_TEMP={target_bed - 5} TIMEOUT=30 CANCEL='_NOTHING'
     {% endif %}
 
+# With the chamber heated, a filament that stays in the toolhead path softens
+# in the PTFE tube (PLA loaded, ASA print). PRINT_START would swap a lane that
+# is not the first tool's anyway, so unload it now, before the chamber heats.
+# AFC moves the filament back into the Box Turtle, outside the chamber.
+[gcode_macro _UNLOAD_FOR_CHAMBER]
+description: Before a chamber soak: unload the toolhead when its lane is not mapped to TOOL (the print's first tool)
+gcode:
+    {% set tool = 'T' ~ params.TOOL|default(0)|int %}
+    {% set cur = printer.AFC.current_load if 'AFC' in printer else none %}
+    {% if cur and ('AFC_stepper ' ~ cur) in printer %}
+        {% set lane = printer['AFC_stepper ' ~ cur] %}
+        {% if lane.map != tool %}
+            RESPOND MSG="Chamber heating: unloading {cur} ({lane.material}, {lane.map}), the print starts with {tool}"
+            _HEAT_FOR_LOADED_LANE TOOL={params.TOOL|default(0)|int}
+            TOOL_UNLOAD LANE={cur}
+            _UNLOAD_FOR_CHAMBER_VERIFY LANE={cur}
+        {% endif %}
+    {% endif %}
+
+# Separate macro: rendered after TOOL_UNLOAD has run. A failed unload stops
+# the print before the chamber heats.
+[gcode_macro _UNLOAD_FOR_CHAMBER_VERIFY]
+description: Helper for _UNLOAD_FOR_CHAMBER: stop the print when the lane is still loaded
+gcode:
+    {% if printer.AFC.current_load %}
+      _ERROR MSG="Chamber heating: {params.LANE} could not be unloaded ({printer.AFC.current_load} still in the toolhead), print stopped before the chamber heats" DISPLAY="Unload failed, print stopped"
+    {% else %}
+      # The filament the retract was remembered for is gone
+      SAVE_VARIABLE VARIABLE=nozzle_retract VALUE=0
+      M104 S150
+    {% endif %}
+
 # Beacon contact refuses a nozzle above contact_max_hotend_temperature (180C)
 [gcode_macro _NOZZLE_CONTACT_TEMP]
 description: Nozzle to 150C for Beacon contact; wait only while it is above MAXIMUM (default 170C), WAIT=1 also waits until it is at least 145C