grt: change 'overflow' to 'congestion'

[eder-matheus]

Summary

Renames every report overflow log/column/marker/comment to congestion across GRT so the vocabulary is consistent between FastRoute and CUGR. Internal C++ symbols (total_overflow_, getTotalOverflowPerLayer(), etc.) and the stable telemetry metric key (global_route__fastroute__overflow_iterations_s) are intentionally preserved.

What changes:

• FastRoute (FastRoute.cpp, maze.cpp, utility.cpp) — Running extra iterations to remove overflow. → ... congestion; the iteration timer label, the GRT-0103 debug header, GRT-0107, the GRT-0230 warning, the RST Tree before overflow iterations / Tree 2D after overflow iterations debug prints, and all Max/Final/Total H/V overflow debug lines. The congestion_report_file CSV column header and the Total overflow: line in that file are renamed.
• FastRoute marker comments — capacity:N usage:M overflow:K → capacity:N usage:M congestion:K.
• CUGR (CUGR.cpp) — Nets with overflow: → Nets with congestion:, Total overflow: → Total congestion:, and the per-3D-edge marker comment's overflow:K (...) → congestion:K (...).
• GlobalRouter (GlobalRouter.cpp) — GRT-0096 final-report column header Total Overflow → Total Congestion.
• Docs / Tcl help (src/grt/README.md, GlobalRouter.tcl) — -congestion_iterations / -tree2D descriptions updated to match.
• Test goldens — 53 .ok files and 3 .rptok files refreshed to reflect the renamed strings. The congestion_markers_cugr.tcl test's regex and comment also updated to expect congestion: in the marker comment.

Type of Change

• Refactoring
• Documentation update

Impact

• All user-facing GRT logs now use a single consistent term ("congestion") for what was previously a mix of "overflow" and "congestion" depending on the call site. The numeric content is unchanged.
• GRT-0096 report column reads Max H / Max V / Total Congestion.
• dbMarker comments produced by both engines read capacity:N usage:M congestion:K (...). The DRC viewer / web inspector pass marker comments through verbatim, and no in-tree tool parses them, so this is a string-level change with no functional impact in the codebase. Downstream scripts (outside this repo) that parse the renamed strings would need to be updated.
• congestion_report_file <file>.rpt output CSV column changed from overflow to congestion; the trailing total line changed from Total overflow: to Total congestion:.
• Telemetry metric key global_route__fastroute__overflow_iterations_s is unchanged so ORFS / dashboard plumbing is not affected.

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 implements a consistent terminology update by renaming "overflow" to "congestion" across the Global Router (grt) module. The changes span documentation, source code logging, report formatting, and test expectations. I have no feedback to provide as there were no review comments to assess.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"