fix: reject empty self_fusion.temperature_spread at config load

[shrdgn]

What & why

SelfFusionConfig.temperature_spread (openfusion/config.py) accepted an empty list with no validation. A config like self_fusion: {temperature_spread: []} — a plausible attempt to disable temperature variation — loads without error, but crashes every self_fusion request deep inside panel.py:

spread = config.self_fusion.temperature_spread
for index in range(config.self_fusion.n):
    temperature = spread[index % len(spread)]   # ZeroDivisionError if spread == []

This adds min_length=1 to the field so an empty list fails fast and clearly at config load (a ValidationError), instead of crashing mid-request with an unhandled ZeroDivisionError.

How it was tested

• ruff check . passes (new code only — two pre-existing formatting diffs elsewhere in the touched files are unrelated to this change and left as-is)
• pytest -q passes (no live network) — ran tests/test_config.py and tests/test_panel.py
• New behavior has a test (test_self_fusion_rejects_empty_temperature_spread)
• Docs updated if config / request surface / defaults changed — no docs describe this field's edge-case behavior, none needed
• No secrets, prompts, or response bodies added to logs or metrics
• Quality/cost claims backed by a reproducible bench/run.py number — not applicable, this is a validation-only fix with no behavior change for valid configs

Notes for reviewers

Found via an automated repo-review scheduled task. Scoped narrowly to the validation gap; no other behavior changes.

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