Split ArcCanvas.jsx into per-component files (#2835)

[atomantic]

Summary

client/src/components/pipeline/ArcCanvas.jsx had grown to ~2875 lines holding 32 top-level components/helpers. This is a pure mechanical, behavior-preserving split: each subcomponent now lives in its own file under client/src/components/pipeline/arcCanvas/, following the one-concern-per-file convention.

• 27 component files + shared.js (holds the SEVERITY_COLORS map used by both VerifyResults and CompletenessResults).
• Single-use constants/pure helpers are co-located with their sole consumer (e.g. VERIFY_ARC_SCOPE in ArcHeader.jsx, ISSUE_STATUS_COLORS/issueCoverRecord in IssueRow.jsx, pickRenderedFilename/pickCoverJobId in CoverArt.jsx, the cover-slot reducers in VolumeCoverLiveUpdates.jsx).
• ArcCanvas.jsx (default export) is now a thin 122-line composer.
• ArcRoadmapChart is re-exported from ArcCanvas.jsx so its existing public import path ({ ArcRoadmapChart } from '.../ArcCanvas', used by PipelineSeriesRoadmap) keeps working. The default export path (used by PipelineSeries, StoryBuilder) is unchanged.

No rendered-output change — component code is copied verbatim; only import depths were adjusted (arcCanvas/ is one level deeper than pipeline/).

Test plan

• cd client && npm test — 416 files / 4555 tests pass.
• npx eslint over ArcCanvas.jsx + arcCanvas/ — clean (no unused/undefined imports).
• cd client && npm run build — production build succeeds, confirming the ArcRoadmapChart re-export and all sibling imports resolve.

Closes #2835