Fix stain-for-reg fallback when spim_templates exists but has no matching stains

[Copilot]

When spim_templates was defined in the template config but none of its keys matched the available dataset stains, stain_for_reg remained None and the workflow raised a ValueError — even though a valid registration stain existed in config["stains_for_reg"]. The fallback to config["stains_for_reg"] was unreachable in this case because it was in an else branch tied to spim_templates is None.

Change

• Replaced else: with if not use_spim_template: so the config["stains_for_reg"] fallback runs whenever SPIM-template matching fails, regardless of whether spim_templates was absent or simply had no overlap with the available stains.

# Before
if spim_templates is not None:
    for stain in spim_templates.keys():
        if stain in stains:
            stain_for_reg = stain
            use_spim_template = True
            break
else:  # ← only runs when spim_templates is None
    for stain in config["stains_for_reg"]:
        ...

# After
if spim_templates is not None:
    for stain in spim_templates.keys():
        if stain in stains:
            stain_for_reg = stain
            use_spim_template = True
            break

if not use_spim_template:  # ← runs when no SPIM template match found
    for stain in config["stains_for_reg"]:
        ...

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