Make conflicting chroma key strategies an error + remove inert options

[hlindset]

Image.Options.ChromaKey has two mutually exclusive strategies, but nothing enforced it.

• select_strategy/1 matched on the merged options map, and the color range clause is defined before the threshold clause, so the color range options always won when both were present.
• because default_options/0 merged [color: :auto, threshold: 20], supplying only one of :greater_than and :less_than fell through to the threshold clause instead, leaving the final clause unreachable.

Image.Options.ChromaKey.validate_options(img,
  color: [0, 255, 0], threshold: 30, greater_than: [0, 200, 0], less_than: [50, 255, 50])
# {:ok, %{greater_than: [0, 200, 0], less_than: [50, 255, 50]}} ← :color and :threshold dropped

Image.Options.ChromaKey.validate_options(img, greater_than: [0, 200, 0])
# {:ok, %{threshold: 20, color: :auto}} ← range discarded, falls back to defaults

Image.chroma_key(img, sigma: 2.0, min_amplitude: 0.05)
# {:ok, image} ← validated, but never used for anything

Changes

• Strategy is selected from the keys the caller supplied, before defaults are merged
• The validated map carries an explicit :strategy key, which do_chroma_mask/2 dispatches on (instead of map key presence)
• Removed :sigma and :min_amplitude. They were never read by anything, and were leftovers from copying Image.Options.Blur (in 1326801)
• Both new errors are %Image.Error{reason: :invalid_option}. The unreachable clause originally returned a bare binary
• Fixed the :greater_than and :less_than docs (which were inverted). The mask covers the range between the bounds, so :greater_than is the lower one

Breaking changes

Affects Image.chroma_mask/2, Image.chroma_key/2, Image.replace_color/2 and their bang variants.

• Supplying options from both strategies returns an error. Previously the range won and :color and :threshold were silently dropped
• Supplying only one of :greater_than / :less_than returns an error. Previously the supplied option was silently discarded and auto chroma detection ran instead
• :sigma and :min_amplitude are rejected. They had no effect, but were accepted