Add seq::derive class for symbolic regex derivatives

Implement a new seq::derive class (seq_derive.h/cpp) that computes
symbolic derivatives of regular expressions using ITE-trees, based on
the RE# approach (Varatalu, Veanes, Ernits - POPL 2025).

Key features:
- Two-argument operator()(ele, r): computes derivative of regex r w.r.t.
  element ele (concrete character or de Bruijn variable for symbolic mode)
- ACI canonicalization (flatten, stable_sort, dedup) for union/intersection
- ITE-tree combinators for binary/unary operations
- Info-based nullability with recursive fallback
- Complement absorption rules
- Depth-bounded recursion to prevent stack overflow

Integration with seq_rewriter:
- mk_derivative(ele, r) and mk_derivative(r) now delegate to m_derive
- Removed dead mk_derivative_rec function
- Added ITE hoisting in mk_re_star, mk_re_concat, mk_re_union0,
  mk_re_inter0, mk_re_complement
- Added depth limiting in Antimirov derivative helpers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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