Prepare release candidate

[ekaf]

Summary of Updates

This pull request prepares the repository for a new release and incorporates the following improvements:

1. Fix for stacked_output_streams (close #14):
   Corrected the defective handling of stacked output streams. Previously, nested tell/told calls could fail on some Prolog systems. The fix ensures that streams are properly closed, reused, or garbage-collected as needed, preventing any unnecessary resource consumption.

2. Optimized Transitive Closure (dynamic visited-set backend):
   Added an alternative transitive-closure backend that tracks visited nodes using a dynamic visited/1 predicate. On Prolog systems with well-indexed dynamic predicates (amortized O(1) lookup/assert), the closure traversal runs in O(E+V) for the reachable subgraph; result collection and cleanup add O(V), so overall remains O(E+V).
   This is compared against the existing ordered-list visited-set approach based on ord_memberchk/2 + ord_insert/3 (from utils.pl portability code / ordsets). For list-based ordsets, membership and insertion are O(|Visited|) worst-case (with early cutoff due to ordering), giving worst-case O(E·V + V²) per closure, though it can still be faster in practice on some systems (notably SWI and Trealla) due to dynamic database overheads.

3. SPDX File Headers for Licensing Compliance:
   Introduced machine-readable SPDX identifiers at the top of all source files to explicitly declare their licensing terms. This change clarifies downstream compliance requirements and aligns with modern practices for open-source repositories.