pad: persist RDL bump routing assignments

[mahjiid]

Summary

Persist explicit assign_io_bump RDL routing intent across write_db / read_db.

Problem

assign_io_bump -terminal and -dont_route are currently stored only in ICeWall::routing_map_. After a database checkpoint/reload that map is empty, so rdl_route can silently route a different topology.

Changes

• Store explicit -terminal targets as a string property on the bump ITerm.
• Store explicit -dont_route intent as a boolean property on the bump ITerm.
• Clear stale persisted intent when a bump is reassigned.
• Teach RDLRouter to restore persisted intent when no in-memory routing-map entry exists.
• Extend rdl_route_assignments with a real write_db -> separate-process read_db round trip before routing.

Validation

• git diff --check passes.
• Commit includes DCO sign-off.
• Regression uses the existing golden DEF, so routing after reload must remain identical to the pre-checkpoint expected result.
• Full OpenROAD CI is left to the project workflow because this host does not have the complete Bazel/C++ toolchain installed.

Fixes #11297

[gemini-code-assist]

Code Review

This pull request ensures that explicit -terminal and -dont_route bump assignments survive database checkpoints by persisting them as properties on the bump terminal. The feedback highlights potential robustness issues when restoring these properties, specifically warning about silent failures if a target terminal is missing or if there is a net mismatch between the bump and its target terminal, and suggests adding defensive checks and warnings.

[gemini-code-assist]

When restoring the explicit routing target from the persisted string property, there are two potential correctness and robustness issues:

1. Silent Failure on Missing Target: If the target ITerm is not found in the design (e.g., due to design changes or renaming), the routing is silently skipped because the else block is bypassed. This should be logged as a warning/error.
2. Net Mismatch: If the restored target ITerm belongs to a different net than the bump ITerm, routing them together would create a short or net mismatch. We should perform a defensive check to ensure their nets match before adding the target to iterm_pairs.

      } else if (auto* target
                 = odb::dbStringProperty::find(iterm, kRdlRouteTargetProperty);
                 target != nullptr) {
        auto* target_iterm = block_->findITerm(target->getValue().c_str());
        if (target_iterm != nullptr) {
          if (target_iterm->getNet() == iterm->getNet()) {
            iterm_pairs.push_back(target_iterm);
          } else {
            logger_->warn(utl::PAD,
                          100,
                          "Persisted RDL routing target {} net {} does not match bump {}/{} net {}",
                          target->getValue(),
                          target_iterm->getNet() ? target_iterm->getNet()->getName() : "NULL",
                          iterm->getInst()->getName(),
                          iterm->getMTerm()->getName(),
                          iterm->getNet() ? iterm->getNet()->getName() : "NULL");
          }
        } else {
          logger_->warn(utl::PAD,
                        101,
                        "Persisted RDL routing target {} for bump {}/{} not found in design",
                        target->getValue(),
                        iterm->getInst()->getName(),
                        iterm->getMTerm()->getName());
        }
      }