fix(engine): data-URI background-image 不再跳过——SVG 图标/壁纸走 fetch+cache（C4 根因）

[leizongmin]

Note

Low Risk
Visibility-only API change with no logic edits; risk is limited to new internal callers if added elsewhere in the crate.

Overview
This PR makes a single visibility change: collect_custom_props in async_load.rs is now pub(crate) instead of module-private.

Behavior of the scanner is unchanged—it still walks CSS text, collects --custom-property: value pairs inside rule blocks, and feeds the existing substitute_css_vars_text path before budgeted render. The change only allows other code in the webview crate to reuse that collector without duplicating the parser (aligned with the bing-t5 C3/C4 work around CSS custom properties and var() substitution).

There are no call-site updates in this diff; runtime behavior should be identical unless another crate module starts calling the function in the same branch.

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(requestId: d232fc72-5496-438f-adb6-6c407af4ce15)