lib_pln: port queue/dedup improvements from benchmark variant

[rTreutlein]

Summary

• port heap-based queue limiting and best-confidence selection into lib/lib_pln.metta
• add incremental dedup helpers and use them during derive expansion to reduce repeated candidates
• replace stamp disjoint cross-product scan with recursive membership checks
• remove debug-style derive noise from the benchmark variant while preserving core behavior

Validation

• ran sh run.sh ./examples/pln_roman.metta as a smoke check

[patham9]

Awesome, thank you very much, this looks great actually!
Any chance we can transfer this PR to https://github.com/trueagi-io/PLN?
A few days ago wet moved lib_pln back there, we didn't yet remove it from PeTTa (removing things not until v1.1 version)

[patham9]

I just tried to override lib_pln of https://github.com/trueagi-io/PLN with the lib_pln of this PR. The more extended test cases there do not pass anymore, but maybe it is minor issues.

[rTreutlein]

I'll have a look at that.

[rTreutlein]

Closing as superseded: the equivalent lib_pln changes have been merged in trueagi-io/PLN.