Remove Transducers.jl dependency

[devmotion]

Closes #200.

Summary

Remove the internal (unexported) Sample transducer and the Transducers.jl dependency, which brought in 11 unique transitive dependencies (Accessors, ArgCheck, Baselet, CompositionsBase, ConstructionBase, DefineSingletons, InitialValues, Markdown, MicroCollections, SplittablesBase, Tables).

The equivalent steps() / Stepper iterator interface remains available for lazy/streaming MCMC sampling.

Options considered

Three approaches were evaluated:

1. Package extension

Move Sample to ext/AbstractMCMCTransducersExt.jl, make Transducers a weak dep.

Pros	Cons
Not breaking for actual users	Still carries maintenance burden for unused code
Eliminates transitive deps for most users	Must still track Transducers compat bounds
Follows existing extension pattern in this repo	Requires constructor-forwarding plumbing for AbstractMCMC.Sample

2. Separate package

Create AbstractMCMCTransducers.jl.

Pros	Cons
Complete decoupling	Highest maintenance overhead (new repo, CI, registry)
	Overkill for ~100 lines of code nobody uses

3. Remove completely (chosen)

Pros	Cons
Simplest change — pure deletion	Removes AbstractMCMC.Sample (internal, unexported)
Eliminates 11 transitive deps immediately
Zero ongoing maintenance burden
steps() provides equivalent functionality
No known downstream users (GitHub search confirmed)
Removes reliance on sparsely-maintained upstream

Rationale: No known users, not exported, equivalent functionality exists via steps(), and the issue author doubts anyone uses it. Option 1 adds complexity to preserve dead code; Option 2 is overkill.

Test plan

• CI passes (all remaining tests should be unaffected)
• using AbstractMCMC loads without error
• Transducers no longer appears in resolved Manifest

🤖 Generated with Claude Code

[github-actions]

AbstractMCMC.jl documentation for PR #201 is available at:
https://TuringLang.github.io/AbstractMCMC.jl/previews/PR201/

[penelopeysm]

Thanks @devmotion! Quite happy to remove it. There are a few remaining references to transducers in the docs, I'll go ahead and remove them too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (0f67b29) to head (6fcf4b5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #201   +/-   ##
===========================
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