Capture inline CSS image-set media before portable export

[chubes4]

Problem

Data Liberation portable captures can omit image assets referenced only by inline CSS url() or image-set(). The frozen Figma Site capture retained those browser references in HTML and localized runtime JSON, but media-stubs.json did not contain the assets. Blocks Engine correctly rejected the import with unresolved_local_browser_reference.

The source later returned 404 for one omitted asset family, so the frozen capture cannot be repaired without fabricating content. This makes capture-time completeness mandatory.

BET-19 evidence

• Studio PR: Route studio create --from URLs through Data Liberation #3952
• SSI policy fix: Fix dangling script preloads in inert imports static-site-importer#925
• Figma artifact compiled after script and fetch-preload fixes, then failed on /_assets/v11/16ea4cf7635cf8568a15abf32c149f41ad2afcb.png?w=1280.
• Both ?w=1280 and ?w=2560 were present in every frozen page as an inline image-set(...).
• Neither URL was present in the capture media store.
• A direct source request now returns HTTP 404.

Expected contract

• Generic media discovery includes image-valued inline CSS url() and image-set() references.
• Responsive variants use the existing bounded family selection policy.
• Portable export rewrites those references in HTML and textual runtime dependencies such as JSON.
• Capture diagnostics report any declared browser reference that could not be localized.

PR #3952 now contains parser and export coverage for these contracts, but the frozen artifact remains permanently incomplete because capture occurred before the fix.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode traced the BET-19 import through DLA, SSI, and Blocks Engine, implemented the capture/export coverage in PR #3952, and drafted this issue. Chris Huber reviewed and remains responsible for the report.

[chubes4]

Closing as redundant tracking. The owning Data Liberation fixes are already implemented on Studio PR #3952 in commits f5d8b06a1, 676ac3e9f, and 7dff40d9c: fetch preload capture, textual runtime-data media localization, inline CSS image discovery, and deterministic single-pass URL rewriting. Remaining BET-19 evidence belongs on that branch/PR rather than a second Studio issue.

AI assistance: OpenAI GPT-5.6 Sol via OpenCode verified the duplicate tracking and drafted this closure. Chris Huber reviewed and remains responsible for the action.