add two-pass soft homing for X/Y and skip rehome on subsequent loading position presses

[hongquanli]

Summary

Adds a two-pass (coarse → back off → slow) homing routine for the X and Y axes to eliminate run-to-run jitter caused by speed-dependent limit-switch trip hysteresis. The "Go to Loading Position" button now only triggers homing on the first press of a session; subsequent presses move via absolute coordinates, avoiding plate-to-plate calibration drift.

What's changed

software/control/_def.py — 5 new MachineConfiguration constants for the soft-homing fine pass: SOFT_HOMING_VELOCITY_X/Y_mm (4.0 mm/s), SOFT_HOMING_ACCELERATION_X/Y_mm, and SOFT_HOMING_BACK_OFF_MM (3.0 mm).

software/control/microcontroller.py

• New soft_home_x() / soft_home_y() / soft_home_xy() methods. Each performs coarse-home → back-off → lowered MAX_VELOCITY → fine-home, with a try/finally covering everything after the velocity is lowered so any timeout in the fine pass still restores the original values.
• New connection_session_id counter, incremented inside attempt_connection() on every successful (re)connect. Used by the navigation layer to invalidate cached "homed" state across firmware position-counter resets.
• A Python-side mirror of per-axis MAX_VELOCITY/ACCELERATION, kept in sync inside set_max_velocity_acceleration so the soft-home save/restore uses the actually-programmed values (not the MACHINE_CONFIG defaults, which may have been overridden by other code).

software/control/core/navigation.py

• New has_been_homed property: True iff a soft-home was committed in the current microcontroller session (compares _homed_at_session_id against the microcontroller's current connection_session_id).
• loading_position_enter gains a with_homing kwarg. With with_homing=False, the function moves via absolute coordinates without re-homing, but refuses to run unless has_been_homed is True (raises RuntimeError).
• Snapshot-and-check guard around the XY homing sequence: wait_till_operation_is_completed auto-recovers from timeouts by calling attempt_connection(), which bumps connection_session_id. If a reconnect happens mid-homing, the homed-state commit is skipped so the next attempt re-homes from scratch.
• Replaces single-pass home_x()/home_y() calls in the loading-position flow with the new soft_home_* variants.

software/control/gui_hcs.py — "Go to Loading Position" button passes with_homing=not has_been_homed. First press of the session homes; subsequent presses skip.

Why

Mechanical limit switches have speed-dependent trip hysteresis: the position at which they activate depends on the approach velocity. The pre-existing single-pass home approached the switch at full MAX_VELOCITY, which introduced run-to-run home-position jitter. Over a session of many plate exchanges, this caused visible plate-to-plate calibration drift.

The two-pass approach (coarse pass at MAX_VELOCITY to find the switch quickly, then a slow second pass for a velocity-independent trip point) removes that dependence. The session-aware skip-rehome removes the trip exposure entirely once a clean home is established within a session.

Test plan

• Manual: with stage in an arbitrary position, click "Go to Loading Position". Confirm full homing routine runs (logs homing - in loading position).
• Manual: click "Go to Loading Position" again from a different stage position later in the same session. Confirm it skips homing (logs no-homing - in loading position, fast move via absolute coordinates).
• Hardware repeatability: measure home-position spread over ~10 consecutive homes; compare to pre-PR baseline.
• Manual reconnect: disconnect/reconnect microcontroller mid-session; confirm next "Go to Loading" press re-homes (does not skip).