Fix PLN test failures from heap queue changes

[rTreutlein]

Summary

• reproduce failures in sh test.sh caused by syntax errors while loading lib/lib_pln.metta
• remove the new heap-based queue path that the current MeTTa to Prolog translator cannot process
• restore LimitSize and confidence selection to the prior BestCandidate-based implementation while keeping other lib_pln improvements

Validation

• before: sh test.sh failed on ./examples/pln_direct.metta, ./examples/pln_roman.metta, and ./examples/pln_tuffy.metta with syntax errors in TaskHeapPush
• after: sh test.sh passes end-to-end

[rTreutlein]

Folded this PR into #127 by fast-forwarding to include commit . Keeping #127 as the single active PR.

[rTreutlein]

Folded this PR into #127 by fast-forwarding lib-pln-queue-dedup-improvements to include commit 3d35b16. Keeping #127 as the single active PR.