Mesh vector refactor part4: Traces and StreamLines

[uclaros]

Description

Following up on the mesh vector refactoring from #67399, #67396 and #67403, this one handles StreamLines and Traces rendering.
This is the last part of the refactoring.

Still no rendering nor user-facing changes

AI tool usage

• AI tool(s): Claude Opus 5 was used during planning and implementing the Traces and StreamLines refactoring.

[github-actions]

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
To execute locally, unzip the downloaded zip file and run bin\qgis-bin.exe in the extracted directory.
You might be prompted by Windows Defender click "Run anyway"
(Built from commit 1deb889)

🍎 MacOS Qt6 builds

Download MacOS Qt6 builds of this PR for testing.
This app is not notarized, run sudo xattr -d com.apple.quarantine /Applications/QGIS*.app to avoid the warning
(Built from commit 1deb889)

🪟 x64-windows — ⚠️ vcpkg failed before producing per-port logs

vcpkg returned a non-zero exit code but no port left a parseable buildtree. This usually means a portfile-execute abort (e.g. find_library REQUIRED, vcpkg_extract_source_archive download failure), or a vcpkg-internal error such as a missing tool or registry/baseline mismatch.

Look at the artifacts and the workflow log:

📑 Download full build-logs-x64-windows artifact · 🔧 Workflow run · 📌 Commit 4276f13