Add compose() for image layer compositing

[kalidke]

Summary

• Adds compose(images...; blend=:additive) utility for compositing pre-rendered images
• Two blend modes: :additive (sum, clamp) and :replace (later non-black pixels overwrite)
• Decouples compositing from rendering — enables mixed-strategy overlays (e.g., Gaussian background + Circle overlay)
• Motivated by BaGoL use case: gray localizations + red BaGoL emitters where additive blending washes out the overlay

Usage

(bg, _) = render(locs, strategy=GaussianRender(), color=:gray, zoom=20)
(fg, _) = render(bagol, strategy=CircleRender(), color=:red, zoom=20)
combined = compose(bg, fg, blend=:replace)

Changes

• src/utils.jl — compose() implementation (varargs + Vector forms)
• src/SMLMRender.jl — export compose
• test/runtests.jl — tests for both blend modes, dimension mismatch, invalid mode
• api_overview.md, docs/src/api.md, docs/src/examples.md — documentation

Test plan

• All 80 tests pass
• Additive blend sums layers and clamps
• Replace blend overwrites non-black pixels, preserves black
• Vector and varargs forms produce identical results
• Assertion errors for dimension mismatch and invalid blend mode

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