pad: add option to mark routes as FIXED in RDL

[gadfort]

Summary

Adds option to mark routes a FIXED, this ensures they will not be ripped up later.

Type of Change

• New feature

Impact

Default behavior is unchanged, this just ensure we have the option.

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 -fixed option to the RDL router, allowing routed wires to be marked as fixed in the database. The changes span across the C++ implementation, SWIG interface, TCL command definitions, and documentation, along with new integration tests. The review feedback highlights a critical backward-compatibility issue: inserting the new fixed parameter in the middle of the default arguments list breaks existing callers. It is recommended to append the fixed parameter to the end of the parameter list across the C++ and SWIG interfaces, and update the corresponding TCL and internal calls. Additionally, a minor typo in the README documentation should be corrected.

[maliberty]

some cleanups needed but otherwise fine