grt: persist routing engine choice for cross-session repair_antennas

[eder-matheus]

Summary

global_route -use_cugr only sets a session-local flag, so in fresh-process flows (e.g. ORFS) a post-detailed-route repair_antennas runs FastRoute on a CUGR-routed design.

This PR saves the engine choice as a block property (grt_use_cugr) in saveGuides and restores it in fresh sessions (loadGuidesFromDB, repairAntennas, startIncremental). An explicit global_route choice always wins, and an absent property keeps today's FastRoute default.

Type of Change

• Bug fix
• New feature

Impact

Fresh-session repair_antennas on CUGR-routed designs now adopts demand and reroutes diode nets with CUGR instead of FastRoute. FastRoute flows are unchanged (repair_antennas_from_odb passes byte-identical; full grt, est, and ant suites pass).

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).

New test: repair_antennas_from_odb_cugr reads a checked-in CUGR-routed db
in a fresh session and asserts the restore (registered in CMake and Bazel).

Related Issues

N/A

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to persist and restore the global routing engine choice (CUGR vs FastRoute) using a database block property. While this is a valuable addition, the current implementation of ensureEngineSelected uses a session-wide engine_selected_ flag that can cause stale state leaks in multi-design sessions. The feedback recommends tracking the last checked block and introducing a user_selected_engine_ flag to safely support multi-design environments and preserve explicit user overrides.

[eder-matheus]

@maliberty Could you comment on the idea of using a dbProperty to store the GRT engine choice? This was based on RSZ's set_opt_config.