Use hardware scan for fine focus instead of step-and-capture

[pskeshu]

Problem

The current fine_focus implementation in gently/agent/tools/focus_tools.py uses a software-controlled step-and-capture approach:

for i, pos in enumerate(positions):
    result = await client.capture_lightsheet_image(
        piezo_position=float(pos),
        galvo_position=float(galvo_position)
    )

This is slow because each iteration requires:

1. Send piezo move command
2. Wait for piezo to settle
3. Trigger camera acquisition
4. Wait for exposure + readout
5. Transfer image data
6. Repeat

For a typical 21-position sweep (±10μm at 1μm steps), this adds significant overhead from communication latency and settle times.

Proposed Solution

Use hardware-triggered scanning where:

• Galvo stays constant at the specified position
• Piezo scans continuously through the range
• Camera acquires a stack triggered by piezo controller

This would use the existing volume acquisition infrastructure but with:

• Galvo held at a single position (not scanning)
• Piezo providing the Z movement
• Camera in triggered/streaming mode

Benefits

• Much faster acquisition (single trigger starts entire sweep)
• Piezo controller handles precise timing
• Reduced communication overhead
• More consistent timing between frames

Implementation Notes

• May need a new client method like capture_piezo_stack() or modify acquire_volume() to support galvo-constant mode
• Focus score calculation remains the same (analyze returned stack)
• Consider if existing acquire_volume can be parameterized for this use case

Related Code

• gently/agent/tools/focus_tools.py - fine_focus() function
• gently/plans.py - existing volume acquisition plans