Widen resolver singletons using adjacent versions

[charliermarsh]

Summary

Every selected registry version currently clones, sorts, and deduplicates the full candidate universe before widening a singleton dependency incompatibility. For the common single-index/no-installed case, use the immediately included neighboring versions directly, skipping cutoff-excluded entries without materializing distributions. Preserve the full-universe path for installed, multi-index, and URL candidates, and reconstruct complete sets before narrowing no-solution diagnostics.

Measurements

Warm offline caches, profiling binaries, pinned CPUs, alternating pairs, and identical lock hashes for every pair:

Workload	Pairs	CPU change (95% paired CI)
Jupyter	200	-1.22% [-1.71%, -0.73%]
Airflow	200	-0.92% [-1.54%, -0.29%]
large-index	200	-1.15% [-1.70%, -0.58%]
Boto	80	-0.27% [-3.22%, +2.76%]

Validation

• cargo fmt --all -- --check
• cargo clippy --locked -p uv-resolver --lib --tests -- -D warnings
• cargo test --locked -p uv-resolver --lib -- --nocapture (66 passed)
• focused exclude-newer, offline/backtracking, invalid-metadata, and no-solution integration tests (7 passed)