MB-71971: Add explain to custom score queries

[capemox]

• Custom score queries with explain: true now have a simple "custom_score function result" message
• Cleaned up custom queries unit tests to use scorch indexes rather than upsidedown

[Copilot]

Pull request overview

This PR adds Explain support to custom score queries by emitting a simple explanation node for the post-mutation score, and updates related unit tests to use a scorch-backed index instead of NewMemOnly.

Changes:

• Thread options.Explain into CustomScoreSearcher and, when enabled, replace the hit explanation with "custom_score function result" (value = final mutated score).
• Refactor CustomScoreSearcher scoring into a helper (applyScore) used by both Next and Advance.
• Update custom query tests to use temporary scorch indexes and add a new explain-focused test.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
search/searcher/search_custom_score.go	Adds an explain flag and wraps the final score in a single explanation node when enabled.
search/query/custom_score.go	Passes options.Explain into the custom score searcher construction.
search_test.go	Switches custom-query test indexes to scorch-backed on-disk indexes and adds an explain test.

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[coveralls]

coverage: 52.292% (-0.01%) from 52.304% — custom_expl into master