feat(graph): intra-stage DAG sub-columns and connectors behind chips

When jobs within the same declared GitLab stage have needs: relationships
between each other, the pipeline graph now splits that stage into
topological sub-columns: jobs with no same-stage deps are in sub-column 0,
jobs that depend on them shift one column right. A new computeColumns()
function handles the topo-sort; a narrower subStageGap (20 px vs 50 px
stageGap) separates sub-columns; stage headers span all sub-columns.

SVG connector lines (Bézier curves in DAG mode, bus-bar stubs in classic
mode) are now emitted before job chip rectangles so connectors visually
pass behind chips rather than on top of them.

100% statement coverage maintained (99 tests in graph package).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -6,7 +6,7 @@
 
 <p align="center">
   <a href="LICENSE"><img src="https://img.shields.io/badge/license-Apache%202.0-blue.svg" alt="License"></a>
-  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.25-blue.svg" alt="Release"></a>
+  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.26-blue.svg" alt="Release"></a>
 </p>
 
 > **Disclaimer:** This tool was built through iterative AI-assisted development with [Claude](https://claude.ai). It is experimental, incomplete, and not intended for production use. Coverage of GitLab CI keywords is best-effort and may lag behind GitLab's evolving spec. Use it at your own discretion — no correctness guarantees are made. Contributions and bug reports are welcome.