fix: guard against TopDocs::with_limit(0) panic

[Copilot]

Summary / 요약

tantivy::TopDocs::with_limit(0) panics at runtime. This is reachable from 4 call sites in src/index/reader.rs — whenever a Count query returns 0 results or the caller passes limit=0.

thread 'main' panicked at tantivy-0.25.0/src/collector/top_collector.rs:85:9:
Limit must be strictly greater than 0.

Repro: cpsurf search pkg "im.toss.teamhome.serialization" on a dependency with no matching documents.

Fix: Early-return empty results when the limit would be 0, at all 4 sites:

• list_packages_for_dependency — Count returns 0 → return (vec![], matched_gavs)
• list_packages_for_gavs — Count returns 0 → return (vec![], vec![])
• search() listing mode — offset + limit is 0 → return empty (vec![], 0)
• search() search mode — sq.limit is 0 → skip TopDocs, still return Count

Checklist / 체크리스트

• cargo fmt -- --check passes / 통과
• cargo clippy -- -D warnings passes / 통과
• cargo test passes / 통과
• If index schema changed: reader/writer are synchronized / 인덱스 스키마 변경 시: reader/writer 동기화 완료
• If new subcommand added: Commands enum updated, main.rs match arm added / 새 서브커맨드 추가 시: Commands enum 업데이트, main.rs match arm 추가