Feature/sets and dicts

[adolgert]

Add SetOfSets utility and fix hierarchical sampler enabled() methods

Summary

Introduces SetOfSets utility type and fixes enabled() interface across samplers to support hierarchical MultiSampler structures.

Key Changes

New: SetOfSets type

• Provides read-only unified view over multiple sets implementing AbstractSet interface
• Avoids memory allocation with lazy iteration
• Handles empty collections correctly (init=0 in length())

Refactored: enabled() for KeyedPrefixSearch

• Moved PrefixEnabled from direct.jl to keyedprefixsearch.jl
• Simplified KeyedRemovalPrefixSearch.enabled() to return keys(kp.index) directly
• Fixed isenabled() - removed keys aren't in index, no need to check values

Fixed: MultiSampler.enabled()

• Changed to collect() generator before passing to SetOfSets
• Fixes type inference when propagators return different enabled set types
• Matches working pattern from MultipleDirect.enabled()

Tests

Added 46 test assertions for SetOfSets covering iteration, set operations, edge cases, and different types. All tests pass including hierarchical MultiSampler scenarios.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 77.60000% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.58%. Comparing base (63345e8) to head (c215f5c).

Files with missing lines	Patch %	Lines
src/prefixsearch/keyedprefixsearch.jl	55.55%	12 Missing ⚠️
src/sample/multiple_direct.jl	72.41%	8 Missing ⚠️
src/sample/combinednr.jl	80.76%	5 Missing ⚠️
src/trace/track.jl	0.00%	2 Missing ⚠️
src/sample/firstreaction.jl	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@                  Coverage Diff                   @@
##           dev/sample-context      #88      +/-   ##
======================================================
+ Coverage               67.37%   68.58%   +1.20%     
======================================================
  Files                      20       21       +1     
  Lines                     843      955     +112     
======================================================
+ Hits                      568      655      +87     
- Misses                    275      300      +25     

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