Fix retraction filter bypass and leading-NOT eligibility parsing

[imaurer]

Summary

• PubTator retraction bypass: is_retracted was hardcoded to false in from_pubtator_search_result, so --exclude-retracted silently passed retracted articles from the PubTator leg. Changed to Option<bool> with None for PubTator rows; unwrap_or(true) conservatively excludes unknowns. Duplicate PMID merge now propagates retraction status from Europe PMC when a PubTator-first row is deduplicated.
• Leading NOT dropped in eligibility expressions: essie_escape_boolean_expression skipped operators[0] when emitting, so inputs like NOT MSI-H became MSI-H, inverting user intent. Fixed to detect and emit the leading unary operator before the first term.

Both fixes include new unit tests. All existing tests continue to pass.

Test plan

• cargo test retract — 6 retraction-related tests pass
• cargo test essie_escape — 3 boolean expression tests pass (including leading-NOT and AND-NOT cases)
• cargo test — all tests pass
• make spec — spec suite passes