feat: add auxiliary part cooling fan support

[ndotzbgt]

Summary

Adds an independently controlled secondary part cooling fan (e.g. the OrcaSlicer "Auxiliary Part Cooling Fan", addressed as M106 P2 / M107 P2), complementing the existing PART_FAN.

Changes

• Hardware configs (config/hardware/fans/): aux_fan.cfg and aux_fan_tachometer.cfg (the latter adds AUX_FAN_TACHO monitoring with the same safety checks as the base fans). Wired pins use the new AUX_FAN / AUX_FAN_TACHO board_pins aliases.
• Control macros (macros/hardware_functions/aux_fan.cfg): START_AUX_FAN / STOP_AUX_FAN, a delayed stop macro, the tachometer background check loop, and an M106 / M107 override that forwards P2 to the aux fan and keeps P0 / bare calls fully compatible with existing slicers.
• Print flow integration:
  • start_print: vent the chamber if it is above the configured temperature before heating, and optionally spread heat during the soak.
  • end_print / cancel_print: optional post-print run-down to cool the bed faster and help recirculating filters/exhaust.
  • control: clean shutdown on PRINT_STOP.
• Variables & docs: new variable_aux_fan_* settings in variables.cfg, include lines in printer.cfg, and AUX_FAN / AUX_FAN_TACHO pinout documentation.

Validation

Added a CI pipeline: tests/lint_jinja.py syntax-checks every templated block with Klipper's own Jinja2 environment, and tests/config_check.py assembles a full reference config and loads it with a real klippy (standard and aux-fan-tachometer variants), run by .github/workflows/validate.yml.

Verified locally: 279/279 templates pass, both klippy config loads pass (only the expected "no MCU connected" step fails), and an injected broken template fails the checks.

Enabling

Uncomment [include config/hardware/fans/aux_fan.cfg] (or aux_fan_tachometer.cfg) in user_templates/printer.cfg, define AUX_FAN (and optionally AUX_FAN_TACHO) pins in mcu.cfg, and tune the variable_aux_fan_* settings in user_templates/variables.cfg.

[mjonuschat]

Thank you for your contribution. I see a few problems with this PR:

1. Separation of concerns, it's doing too much. You've added the aux fan feature, a jinja template linter, a config checker and a CI pipeline under the guise of the aux fan. These should should be at least two, maybe three PRs. Especially given how heavy the config checker is.
2. There's a number of bugs in the aux fan implementation:
   1. fan monitoring seems to get disable before later end print block that runs the fan for a while
   2. The M106 rename only works when the user included a [fan] block - that is a separate and optional component - hotends like the Chube conduction don't require one, so this is brittle
   3. aux_fan_soak_speed seems to conflict the the hardcoded min_rpm
   4. there's a disconnect between _MODULE_HEATSOAK_BED and _MODULE_HEATSOAK_CHAMBER - they are independent, the fan might never get stopped
   5. CURRENT_CHAMBER_TEMP > aux_fan_vent_temp triggers on room temp with the current defaults, leading to a 2 min delay for everyone, no matter if they opted in.
   6. the start routinge also seems to skip tach monitoring

[mjonuschat]

Hey, it looks like there are still some of the bugs I mentioned earlier open in this pull request and the new design with every file that has a [fan] section requiring an include is quite brittle and a maintenance burden for every future board we want to add.

And same as the other changes you had in before, the axis twist compensation needs to be in a separate PR