Correct Bayesian score fusion semantics and pruning

[jaepil]

Description

This fixes probability semantics and competitive pruning for BayesianScoreQuery and LogOddsFusionQuery.

The previous fusion path applied softplus gating by default, which discarded negative log-odds evidence, and could propagate competitive-score pruning to individual signals. Pruning an individual negative signal changes matching negative evidence into a neutral non-match and can incorrectly increase the fused score. Single-active-signal leaves and MatchNoDocsQuery rewrites could also bypass or change the declared fusion model.

The change:

• preserves signed log-odds internally while returning a non-negative sigmoid probability
• keeps softplus gating as an explicit opt-in and retains existing constructor signatures
• supports per-signal prior removal and a single target prior
• preserves neutral signals across rewrites and single-active-signal leaves
• applies outer boosts after Bayesian calibration and fusion
• performs block-max skipping only from a safe upper bound on the final fused score
• makes BayesianScoreEstimator use all matching live documents rather than a capped top-doc sample based on maxDoc

Validation

• ./gradlew :lucene:core:test --tests org.apache.lucene.search.TestBayesianScoreQuery --tests org.apache.lucene.search.TestLogOddsFusionQuery
• ./gradlew :lucene:core:check -x checkLicenses (8,548 tests passed, 309 skipped)
• ./gradlew checkLicenses
• ./gradlew :lucene:core:tidy :lucene:core:javadoc
• uv run --project dev-tools prek run --files <changed files>