Purge check: fail when bd_pressure does not answer, add override

A bd_pressure that does not answer used to be reported as "pressure
skipped" and the purge passed unchecked. It now fails the check, so in
enforce mode the printer pauses (and a resume purge stays paused).
PURGE_CHECK_IGNORE_SENSOR lets purges pass without the sensor until the
print ends; _PRINT_CLEANUP clears it so the next print checks again.

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

README.md

@@ -34,7 +34,7 @@ everything below, grouped by topic. Comment out what you do not use there.
 | **afc/** | | |
 | `xy_safety_check.cfg` | `AFC_XY_SAFETY_CHECK`, `_AFC_STORE_HOME_REFERENCE`, `AFC_XY_RESUME`: lost-step check around tool changes | `extras/xy_drift.py` (CoreXY) |
 | `lane_tools.cfg` | `LANE_FEED_UNTIL_HUB_OPEN`, `LANE_FEED_ABORT` | AFC |
-| `purge_check.cfg` | `BLOBIFIER_CHECKED`: checks every AFC purge with bd_pressure (and optionally bdwidth motion) | Blobifier, AFC, `extras/bdpressure_check.py` + bd_pressure load-check firmware, `extras/bdwidth_status.py` |
+| `purge_check.cfg` | `BLOBIFIER_CHECKED`: checks every AFC purge with bd_pressure (and optionally bdwidth motion); a bd_pressure that does not answer fails the check, `PURGE_CHECK_IGNORE_SENSOR` lets the current print finish without it | Blobifier, AFC, `extras/bdpressure_check.py` + bd_pressure load-check firmware, `extras/bdwidth_status.py` |
 | `purge_until_empty.cfg` | `PURGE_UNTIL_EMPTY`: purges a snapped filament out until there is no more nozzle pressure | same as `purge_check.cfg` |
 | **pressure_advance/** | | |
 | `bd_pa_spoolman.cfg` + `bd_pa_*.sh` | `LANE_CALIBRATE_PA`, `PA_CALIBRATE_ALL_LANES`, `LANE_RECAL_PA`, `SET_SPOOL_ID`, `PA_STATUS`: pressure advance per lane/spool, cached locally and in Spoolman; a calibration with no result, a pause during the test or too few/stuck sensor readings is rejected and not saved | bd_pressure, AFC, Spoolman, `gcode_shell_command` |