Add open variable sequences

[zayenz]

This adds open variable sequences for models whose eventual length is decided during propagation and search. A length variable controls materialization, and factories and transition callbacks act in the current space.

The change includes:

• A shared OpenVarSequence<Var> core, with integer and Boolean constructors and custom factories for other variable types.
• Integer constraints for distinctness, adjacent relations, sliding count and sum windows, minimum/maximum, and precedence.
• Regular-language constraints for integer and Boolean sequences that accept extendable prefixes and require acceptance when the sequence closes.
• Standard Gecode branching selectors, tie-breaking, filters, print callbacks, and no-goods, with length-first or value-first order. Action and CHB selectors require the complete sequence to be materialized when posting.
• Compact numbering for sparse DFA state identifiers, including duplicate final-state handling.

Validation:

• Debug build and configured CTest checks passed. The final clone cleanup was rebuilt and covered by the focused tests.
• All 39 focused open-sequence, open-regular, DFA, and NFA tests passed twice, including a regression for no-good extraction after sequence growth.
• Independent full-branch review completed; identified issues were fixed and independently checked.
• Allocation-failure cleanup was checked statically; fault injection was not run.
• All 14 cross-platform CI checks passed for commit 934e9b691e.