rsz: BufferToInverters move

[erendn]

Summary

This PR introduces a new inverter buffering move, which replaces a buffer with two inverters uniformly placed along the fanout (--(1/3 wire)--> inv1 --(1/3 wire)--> inv2 --(1/3 wire)-->).

Type of Change

• New feature

Impact

The new move isn't enabled by default. It can be enabled by adding it to the move sequence, like repair_timing -sequence "unbuffer vt_swap sizeup swap buffer clone split buffer_to_inverters". I have run public CI benchmarks and have seen an average improvement when this move is appended to the default sequence in post-CTS and post-GRT timing repair.

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 introduces a new optimization move, InvBuffer, which replaces a single buffer instance with a pair of cascaded inverters to reduce wire delay. The implementation includes the new InvBufferCandidate and InvBufferGenerator classes, integration into the optimization policies, CLI/Python interface updates, and new unit tests for flat and hierarchical designs. The review feedback highlights several critical improvement opportunities: resolving a logical bug where footprint matching prevents finding valid inverter replacements when -match_cell_footprint is enabled, handling block terminals (BTerms) to ensure accurate physical placement when drivers or loads are ports, and adding defensive nullptr checks on instance creation to prevent potential crashes.

[gemini-code-assist]

In InvBufferGenerator::generate, the footprint matching check compares the footprint of the original buffer (drvr_footprint) with the footprint of the candidate inverter (cell->footprint()). Since buffers and inverters have different functions, they will almost always have different footprints in Liberty libraries (e.g., buf vs inv). If -match_cell_footprint is enabled, this check will prevent the generator from finding any valid inverter replacement, effectively disabling the InvBuffer move. This check should be removed, or replaced with a Vt-matching check if the goal is to match the threshold voltage class.