Visual/screenshot capture convergence (one before/after preview path)

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Part of #983.

Context

Two visual-capture paths exist and must reconcile into ONE before/after preview path. reviewbot has core/browser-render.ts (headless HTML render via the Browser binding, degraded-fetch fallback) + agents/gittensory/{capture,shot}.ts (the taopedia-style realtime before/after screenshot pipeline: route resolution, mobile viewport, authed-route preview-session via ?preview=1/VITE_PREVIEW, the preview-session param). gittensory has src/visual-agent/visual-diff.ts (pixelmatch/pngjs, Node-only, NOT wired to any Browser binding) — so capture is a real gap while the diff is native. The Browser binding is added in #1017. Authed /app/* routes previously showed sign-in walls; the existing synthetic preview session + an honest auth-wall placeholder are the fix.

Deliverables

• Reconcile reviewbot's browser-render/capture/shot with gittensory's visual-agent/visual-diff into ONE before/after preview path (capture from reviewbot, diff from gittensory).
• Wire the Browser binding (coordinated with Infra bindings + adapters (LOCK DO, Vectorize, R2, Browser, private-config home, queue) #1017) for the capture step; fail-safe to no-preview / degraded-fetch when absent.
• Preserve authed-route previews: enable the synthetic preview session on preview builds (VITE_PREVIEW + ?preview=1), authedRoutePattern/previewSessionParam, and the honest auth-wall placeholder when a route still walls.
• Render before/after into the unified comment's capture table (parity with the existing preview surface).
• Add tests: capture resolves routes + mobile viewport; diff produces a before/after; missing Browser binding degrades to no-preview.

Acceptance criteria

• A single before/after preview path produces capture (Browser) + diff (pixelmatch) for visual PRs.
• Authed /app/* routes preview via the synthetic session, not a sign-in wall; walled routes show the honest placeholder.
• Missing/failed Browser binding degrades gracefully to no-preview, never errors the review.

Notes

• Milestone: M1
• Depends on: Infra bindings + adapters (LOCK DO, Vectorize, R2, Browser, private-config home, queue) #1017
• Reconciles reviewbot capture with gittensory diff; the Browser binding lands with this feature, not before.

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Assessment (convergence): visual-capture is a net-new feature, not a wiring/reconcile — deferring.

Investigated reconciling reviewbot's before/after PR screenshots into the unified review comment. Findings:

• gittensory has no screenshot-capture pipeline (buildCapture does not exist in the worktree). The only visual code is a pure pixel-diff util (src/visual-agent/visual-diff.ts) that consumes PNG buffers but never captures them; nothing in src/ calls it.
• The unified review comment has no visual/before-after section.
• test/unit/worker-entry-boundary.test.ts forbids visual-agent//pixelmatch/pngjs/visual-diff from the Worker entry; the unified comment is Worker-reachable, so it cannot import the diff util.

A real 'visual capture into the unified comment' needs 3 net-new pieces (none present): (1) a Node-agent Browser-Rendering/Playwright capture entry producing before/after buffers from the base UI + the per-PR preview deploy (.github/workflows/ui-preview*.yml already record environment_url); (2) R2 (REVIEW_AUDIT) persistence + a public-safe image URL so the comment can ![](…); (3) a boundary-safe surface — the comment receives pre-rendered image URLs/markdown computed agent-side, never importing visual-diff, keeping worker-entry-boundary green.

Reclassified: its own multi-part feature (flag REVIEWBOT_VISUAL, default OFF, wired through UnifiedCommentContext as a pre-rendered collapsible), not part of M1 wiring. Deferred — does not block the cutover; text/gate review is complete.

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Closing as completed (2026-07-01 issue audit): Visual capture converged to one before/after path (src/review/visual/capture.ts + visual-wire.ts, #980/#1490/#1536).