feat(spark): add spectrum scan support for absorbance and fluorescence

[xbtu2]

Summary

Add read_absorbance_spectrum() and read_fluorescence_spectrum() methods to ExperimentalSparkBackend, enabling wavelength-sweep spectral scanning on the Tecan Spark plate reader.

Key Design Decision

The Spark firmware natively supports wavelength range sweeps via WAVELENGTH=FROM~TO:STEP syntax in its filter commands. This implementation leverages the native sweep rather than issuing per-wavelength commands, which is both faster and simpler.

Changes

controls/optics_control.py

• wavelength param type: Optional[int] → Optional[Union[int, str]] to accept firmware range strings (e.g. '5000~5500:100')

spark_backend.py

• Added read_absorbance_spectrum() and read_fluorescence_spectrum() public methods
• Extracted shared helpers to reduce duplication:
  • _run_measurement(): shared background-read → scan → cleanup orchestration
  • _setup_absorbance() / _setup_fluorescence(): shared instrument configuration

spark_processor.py

• Added process_absorbance_spectrum() and process_fluorescence_spectrum() for multi-wavelength TDCL data
• Extracted shared helpers:
  • _find_key(): dynamic TDCL key lookup (replaces repeated next() patterns)
  • _extract_measurement_pairs(): unifies nested_mult and flat block handling
  • _extract_fluo_calibration(): shared dark/bright/K extraction
  • _reshape_to_rows(): shared wavelength→rows×cols reshaping
  • _get_dark_for_wl(): per-wavelength dark value lookup with fallback

Tests

• 14 new tests covering spectrum scan methods and parameter validation
• All 25 tests pass

Physical Hardware Validation

Tested on a physical Tecan Spark via USB/IP forwarding:

Scan	Range	Result
Absorbance spectrum	500–550nm, 10nm steps	6 wavelength points, OD ≈ 0 (empty well) ✅
Fluorescence spectrum	ex 440–480nm → em 545nm	5 wavelength points, 353→213 RFU ✅

[rickwierenga]

cool, thanks for the PR!

in another PR (#1033) we named the spectral scans experimental_ since we are still figuring out the PLR abstraction for those methods. This way, people can use them but it's clear that the API might change.

[xbtu2]

Good call — renamed both methods to experimental_read_absorbance_spectrum and experimental_read_fluorescence_spectrum to match the convention from #1033. Pushed in 013c8fb.

[rickwierenga]

sweeeeet