Atlas companion frontend: integration proposal for #1860

[BernhardJackiewicz]

Context

This follows the discussion with Martin about making a deliberate size exception for the Atlas work. I am opening the required design/tracking issue before proposing any upstream pull request.

I built a separate companion frontend against the current feat/atlas-r1 implementation from #1860. The work does not modify the CBM checkout yet, and I do not consider it merge-ready without maintainer review.

What exists

The companion currently includes:

• a read-only Monaco source reader backed by the CBM read surface;
• a semantic twin that follows the symbol under the caret;
• flow, entry-point, bug-hunt and change-scope reading modes;
• deterministic, source-cited answers with optional local-model wording;
• a persistent graph view with bidirectional focus following;
• live-agent visualisation from a local replay/bridge;
• explicit air-gap, provenance and index-limit messaging;
• an extensive offline acceptance and browser verification harness.

The exact reviewed candidate is BernhardJackiewicz/CodeAtlasWeb@1825f9c850ac794066e02606012508b9d393c6aa. The repository is private while licensing, provenance and the intended upstream shape are reviewed; a maintainer-only write invitation is being provided separately; no other external collaborator has write access.

Why this needs an architecture decision

The implementation is currently a standalone SPA consuming the read-only interfaces exposed by #1860. It has not been folded into graph-ui, and CBM currently has no documented source-level home for a second SPA under --ui.

Before preparing an upstream branch, I would like the maintainer to choose one of these directions:

1. integrate the useful surfaces into the existing graph-ui and shared Atlas chrome;
2. establish an official second-SPA/static-assets route under --ui;
3. keep this as a separately versioned companion project and upstream only focused CBM fixes/read-surface improvements.

Proposed contribution shape after that decision

• Base any stacked draft PR on feat/atlas-r1 while CBM Atlas: the human interface to the code graph #1860 remains open, then retarget/rebase after CBM Atlas: the human interface to the code graph #1860 lands.
• Keep CBM backend fixes separate from frontend product work.
• Transfer only upstream-relevant source, tests, documentation and license material; do not import the full private proof history or large recorded artifacts.
• Recreate upstream commits with conventional subjects and DCO sign-off.
• Keep the resulting PR in draft until Martin has reviewed the architecture and scope.

Current verification

The current companion candidate has passed:

• 2,024 unit tests;
• 213 frozen acceptance checks;
• a fresh two-model local evaluation and release gate;
• an independent re-audit with no remaining product/code release blocker;
• a 30-state Playwright interaction, text-plausibility and visual review with no findings, browser errors or outbound requests.

Those results demonstrate the review candidate; they are not a request to waive upstream review or CBM's own CI/security requirements.

Review requested

Martin: please choose the preferred integration direction above, or propose another one. Once that is settled, I will prepare the smallest reviewable DCO-signed draft contribution rather than treating the current standalone repository as a presumptive merge candidate.

Related: #1860