euf_seq_plugin: root-canonical simplification fixes + loop bound overflow guard

[Copilot]

euf_seq_plugin was applying several regex concat simplifications against raw child enodes instead of canonical e-graph roots, so rules could silently miss after merges. It also merged loop bounds with unchecked unsigned addition, allowing overflowed bounds.

• Root-canonical checks in simplification rules

  • Use get_root() for nullable/full-seq absorption checks.
  • Update same_star_body to test is_star on roots and compare rooted star bodies.
  • Evaluate extended star rules (v*.v*.c / c.v*.v*) against concat roots, not syntactic children.

• Safe loop merge arithmetic

  • Gate loop merging on overflow-safe bound addition checks before constructing merged re.loop.

• Re-simplify concats affected by child merges

  • In propagate_merge, re-run propagate_simplify for tracked concat nodes whose left/right child root is in the merged class, not only concats in the class itself.

• Regression coverage in src/test/euf_seq_plugin.cpp

  • Added focused tests for:
    • star merge firing after child-to-star merge,
    • extended star rule using root concat shape,
    • nullable absorption via merged roots,
    • loop-merge non-application on overflow.

// before: structural check on non-canonical enode
if (is_concat(b, b1, b2) && same_star_body(a, b1))

// after: structural check on canonical representative
if (is_concat(b->get_root(), b1, b2) && same_star_body(a, b1))