Docs: corrected coolant curve and the rebuilt flow check

Records why the temperature formula changed (and that everything
derived from the old one had to be re-derived), the flow
characterization data at both heater duties, the false negative that
killed the dT-threshold design, and the downstream-rise check that
replaced it. Adds flow_characterize.py to the bench kit.

scripts/bench/README.md

@@ -12,6 +12,8 @@ target board (dev image, python3 present) unless noted.
 | `pwm_hold.py` | Holds one PWMSAR value for a scope-measurement window (`pwm_hold.py <sar> <seconds>`), then restores. Same locked-state rule. |
 | `fire_test.py` | FIRE drop-timing scope test (runs on the board): A = latch locked (expects nothing on FIRE/LASER_ON), B = latch unlocked / normal end-of-data, U = true underrun. Duty 0 throughout; refuses to unlock if HV reports good. All gates PASSED with it 2026-08-02 (2.0000 s pulses exact, both paths). |
 | `fan_test.py` | Fan/coolant bench (Windows-side): snapshots fan PWMs/tachs/temps, drives M8 → cut fans, M9 → cooldown → idle, verifying via tach readbacks. All-green 2026-08-02. |
+| `flow_characterize.py` | Coolant flow characterization using the factory temperature curve: baseline → flow → no-flow → recovery, printing the ΔT bands and their separation. Takes the heater duty as an argument (`flow_characterize.py 30`); aborts if downstream passes 45 °C. This is what showed the 10 % scheme was unusable (0.04 °C gap) and sized the 30 % check. |
+| `temp_calibrate.py` | Coolant temperature calibration helper (`watch` / `point <measured_C>` / `fit`) — pairs a measured temperature with averaged raw ADC readings and fits the line. Used to sanity-check the factory curve against a thermometer. |
 | `build-glowforge.sh` | Cross-compiles **grblHAL-glowforge** (the canonical driver repo, `../../../grblHAL-glowforge`) in the forge-yocto WSL distro. Run: `wsl -d forge-yocto -- bash <path>/build-glowforge.sh` (from PowerShell; Git Bash mangles /mnt/c paths). This is the production controller build. |
 | `build-feeder.sh` | Cross-compiles `feeder.c` the same way. |
 | `puls_profile.py` | Decodes factory `.puls` streams (raw or GF1-headered) into velocity/accel profiles: peak speeds, ramp-slope fits, per-move segments, Z cadence. Runs anywhere (stdlib only). Source of the factory-true grblHAL defaults (milestone 2): 700/590 mm/s² accel, 200 mm/s max rate, 28160 Hz travel tick. |