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

Read RegexSpec.lean, starting with regex_match_sound and the Glushkov proofs, then compare the specialized lemmas with the general lemmas in Matcher.lean. Review the DecidableEq branches and the discarded dr binding; done means the comments, robust proof cases, and general-lemma reuse are applied without changing proof behavior.