ATC: split tool-change M-codes into composable atoms

Match auxcnc firmware v3, which dropped the monolithic DROPTOOL /
GRABTOOL ESP tasks in favour of three atoms: RELEASE, CLAMP, EJECT.
This lets host macros interleave Z moves between ejector pulses
(the old DROPTOOL ran open->oscillate->clamp in a single ESP task,
so you couldn't lift Z mid-eject).

  AuxAxis: replace atc_droptool() / atc_grabtool() with atc_eject(
    pulse_ms=, dwell_ms=). atc_release() / atc_clamp() are unchanged.

  Ctrl: register internal hooks for release / clamp / eject only.
    The eject hook parses 'pulse=' and 'dwell=' kwargs out of the
    HOOK:eject:<data> payload so macros can emit
    (MSG,HOOK:eject:pulse=400 dwell=300) for tuned wiggles.

  AuxPreprocessor: M100 now maps to eject (was droptool); M101 is
    unmapped (was grabtool, now a pure host-side macro); M102/M103
    are unchanged. Header comment updated.

  docs/AUX_A_AXIS.md: mention the new atom set.

The drop.nc and grab.nc gcode macros on the controller are
correspondingly rewritten on-device as compositions:
  drop = M102 + 4xM100 + G53 G0 Z0 + M103
  grab = M102 + G4 P2 + M103

docs/AUX_A_AXIS.md

@@ -8,8 +8,9 @@
 > blended with XYZ in the same S-curve plan and the gcode surface
 > below applies as plain `A` words.
 >
-> The HOOK pipeline still exists for ATC pneumatics (M100..M103),
-> see `bbctrl/AuxPreprocessor.py`.
+> The HOOK pipeline still exists for ATC pneumatic atoms (M100 EJECT,
+> M102 RELEASE, M103 CLAMP) - see `bbctrl/AuxPreprocessor.py`. Macros
+> compose drop/grab tool sequences from those atoms.
 
 This adds a virtual `A` axis to the bbctrl controller, driven by the
 auxcnc ESP32 over USB serial (`/dev/ttyUSB0`). The ESP owns step-pulse