Expand engine test matrix

[krwg]

Summary

Audit gaps in vitest coverage for ranking, filters, embeddings, edge cases.

Target milestone: 2.8.5
Work branch: 2.8.5/issue-7-test-matrix

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Problem

Missing tests for corpus cap, semantic boost, filter matrix, snippet case, ONNX mock, OR groups, ollama/corpus-idb smoke.

Expected behavior

Regression net covers all audit-identified engine gaps.

Suggested approach

Add focused test files; extend ranking snapshots deliberately when scoring changes.

Files / areas

• test/ranking.test.js
• new test/engine-filters.test.js
• test/embeddings.test.js

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Acceptance criteria

• Each audit gap has at least one test
• CI green

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Metadata

Field	Value
Priority	P1
Milestone	2.8.5
Branch	2.8.5/issue-7-test-matrix