pdn: prefer BTerms of same net name and avoid finding bterms for each shape

[gadfort]

Summary

When multiple exisiting bterms are present on a net, prefer the one with the same name, otherwise fall back to the first one.
Also moves the bterm creation to the start of the database writing instead of at each shape (no impact but avoids a bunch of name lookups at the inner most part of the writing.

Type of Change

• Bug fix
• Breaking change (bterm name check)

Impact

Avoids randomly selecting a bterm of a different name when one with the same name is available.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have included tests to prevent regressions.
• I have signed my commits (DCO).

[gemini-code-assist]

Code Review

This pull request refactors how boundary terminals (dbBTerm) are handled and created during PDN generation. Instead of dynamically finding or creating dbBTerm instances inside individual shape-writing routines, they are now pre-allocated and mapped in PdnGen::writeToDb and passed down through the grid and grid components to the shapes. Additionally, any unused empty dbBTerm instances are cleaned up at the end of the database write. A new integration test has been added to verify behavior with multiple boundary terminals on a net. There are no review comments to address, and I have no additional feedback to provide.

[maliberty]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

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[maliberty]

Please address

11:36:32  The following tests FAILED:
11:36:32  	6904 - ram.make_7x7_nangate45.tcl (Failed)               IntegrationTest tcl ram log_compare
11:36:32  	7524 - pdn.core_grid_with_multiple_bterms.tcl (Failed)   IntegrationTest tcl pdn log_compare

before mergin