Adds constraint-type defaults, partitioner warnings, and model dtype passthrough

[pkuchmiichuk]

Description

Change	Files
ListConstraint subclasses now default constraint_type to their own discriminator value, so constraints no longer have to restate it at construction (UniquenessConstraint(property_expression=...) instead of also passing constraint_type="uniqueness").	bead/lists/constraints.py
ListPartitioner now warns once per expression when a constraint property expression cannot be evaluated, and once per type when a constraint type is not enforced during assignment. Previously a misconfigured constraint scored zero on every iteration and was silently ignored.	bead/lists/partitioner.py
Causal LMs load in the checkpoint's own dtype by default (dtype="auto"), rather than being forced to float32. Loading a half-precision checkpoint as float32 doubled its memory.	bead/items/adapters/huggingface.py
The requested dtype is threaded from LanguageModelScorer through to the adapter.	bead/items/scoring.py

All four are backward compatible. The new dtype parameters are keyword arguments defaulting to "auto", and the constraint defaults only add a value where one previously had to be supplied.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• Tests (adding or updating tests)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have run uv run ruff check . and uv run ruff format .
• I have run uv run pyright with no errors
• I have added tests that prove my fix/feature works
• All tests pass (uv run pytest tests/)
• I have updated documentation as needed

Testing

Test file	Covers
tests/lists/test_constraints.py	Each constraint type constructs without an explicit constraint_type, and the discriminator still round-trips.
tests/lists/test_partitioner.py	The unresolved-expression warning fires once per expression.
tests/lists/test_partitioner_batch.py	The unenforced-type warning fires once per constraint type.
tests/items/adapters/test_huggingface.py	The adapter forwards dtype to from_pretrained and defaults to "auto".
tests/items/test_scoring.py	LanguageModelScorer passes its dtype through to the adapter.