test(search): cover StartLine intent tie breaks

[tae2089]

Summary

• add a regression test for intent-score ties where two declarations share every identity field except StartLine
• exercise both seed orders and assert the same answer order
• prove the test fails when CompareIdentity stops comparing StartLine

Validation

• CGO_ENABLED=1 go test -tags "fts5" ./... -count=1
• make search-eval
• ccg build .
• ccg docs --out docs
• ccg lint

Closes #112