Refactor addon system to annotations

[bvdmitri]

Replaces the old addon system with a simpler, more efficient annotations system.

Highlights

• Message and Marginal now carry an AnnotationDict (lazy-allocated) instead of a typed addons tuple. Type parameter A removed: Message{D, A} → Message{D}.
• Rules no longer return (result, addons) tuples — just the result. The @Logscale macro writes directly into the dict via annotate!.
• Annotation processors (AbstractAnnotations subtypes) handle post-rule and post-product processing externally via post_rule_annotations! / post_product_annotations!.
• Built-in processors: LogScaleAnnotations (replaces AddonLogScale) and InputArgumentsAnnotations (replaces AddonMemory).
  AddonDebug is removed — use callbacks instead.
• Shims: AddonLogScale() and AddonMemory() throw descriptive errors pointing users to the replacements.
• @call_rule: return_addons / addons keyword removed — pass an AnnotationDict via the annotations keyword and read it back.

Docs

• New "Annotations" section: overview, log-scale, input arguments
• New "Migration guides" section with a v5 → v6 guide
• CHANGELOG updated

[github-actions]

🤖 Code Formatting

Your PR still has some code formatting issues. I've updated PR #597 with the necessary formatting changes.

You can merge that PR into this branch to fix the code style check.

Alternatively, you can run make format locally and push the changes yourself.

[codecov]

Codecov Report

❌ Patch coverage is 92.47312% with 14 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-6@b8397a7). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/message.jl	77.27%	5 Missing ⚠️
src/ReactiveMP.jl	50.00%	1 Missing ⚠️
src/annotations.jl	97.50%	1 Missing ⚠️
src/marginal.jl	88.88%	1 Missing ⚠️
src/nodes/predefined/delta/delta.jl	0.00%	1 Missing ⚠️
src/nodes/predefined/delta/layouts/default.jl	0.00%	1 Missing ⚠️
src/nodes/predefined/distribution/distribution.jl	0.00%	1 Missing ⚠️
src/rule.jl	92.85%	1 Missing ⚠️
src/rules/mixture/out.jl	0.00%	1 Missing ⚠️
src/variables/data.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             release-6     #596   +/-   ##
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  Coverage             ?   81.73%           
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  Files                ?      211           
  Lines                ?     6423           
  Branches             ?        0           
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  Hits                 ?     5250           
  Misses               ?     1173           
  Partials             ?        0           

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