feat: BM25/IEF-style lexical weighting for element discrimination

[Djain912]

Summary

• add \ElementFrequency\ with \Build()\ and \IEF()\ for per-snapshot token discrimination
• compute frequency stats once per lexical \Find()\ call and reuse for all element scoring
• add \LexicalScoreWithFrequency(query, desc, ef)\ while keeping \LexicalScore\ behavior unchanged for nil weighting
• apply IEF weighting to weighted Jaccard intersection/union to reduce impact of common tokens like 'button' and 'link'
• add tests for rare-vs-common IEF behavior, nil backward compatibility, and checkout-button discrimination in a many-button snapshot
• add a 200-element lexical benchmark case
• fix E2E test fixture: 'table of contents' now correctly ranks e3 (link: contents) over e9 (heading: contents) due to IEF downweighting common 'link' role token

Issue

Closes #3

Validation

• \go test -c ./internal/engine\ (compile-only validation)
• E2E Docker tests: all 123 tests passing (fixture updated for IEF ranking change)
• Full benchmark execution is blocked in this environment by Windows Application Control policy for generated test executables

[luigi-agosti]

@Djain912 fix the conflict and feel free to merge this pr