style: proof style improvements in RegexSpec.lean

Items

Low-priority style items flagged by AI code review on PR #9.

1. Unused dr binding in regex_match_sound

regex_match_sound destructs the regex_match_correct result with obtain ⟨ast, _, h_parse, _, h_match⟩ — the _ discarding dr is fine. However, a comment noting why dr is intentionally discarded (we only need the AST-level match) would help readers understand the relationship.

2. Fragile DecidableEq instance comparison

Some proof branches use pattern matching on DecidableEq results that could be made more robust with cases, simp, and Subsingleton.elim to avoid depending on instance resolution order.

3. Specialized lemmas that could use general ones

Some lemmas in the Glushkov submodule have specialized proofs for things that general mathlib lemmas already cover. Replace with calls to general lemmas from Matcher.lean.

References

• Flagged by AI code review on PR #9

RegexSpec.lean を読み、regex_match_sound と Glushkov の証明から始めて、その後、特殊化された補題と Matcher.lean の一般的な補題を比較してください。DecidableEq の分岐と破棄された dr binding を確認してください。コメント、堅牢な証明ケース、一般的な補題の再利用を、証明の挙動を変更せずに適用できていれば完了です。