Greedy join ordering for query optimization

[bplatz]

Improves query :where clause optimization by using greedy join ordering within optimizable segments and carrying forward in-scope variables across segments to prefer joins over cartesian expansion.

For queries with join patterns across references, this improves performance by an order of magnitude, in fact I'd consider this fix a bug that was introduced with the original optimization code as it makes certain types of queries less efficient than they likely were, however now they are faster with this fix.

Key changes

• Greedy join ordering: prefers patterns that share variables with the current in-scope set, then chooses the lowest-selectivity pattern (stable tie-breaker).
• Cross-segment var carry-forward: maintains an approximate “vars in scope” set as segments are processed.
• No behavior change intended beyond improved ordering; same optimizer metadata is preserved for explain output.

[zonotope]

I think there is a little overlap with #1182 and https://github.com/fluree/db/compare/feature/inline-multivariable-filters...feature/reorder-high-level-patterns?expand=1, but I can resolve the conflicts there.

lgtm