Update v0.25.1 README diagrams

[zzhang82]

What changed

• redesigned the README-linked amb-overview.svg around the v0.25.1 runtime, authority, trustworthy retrieval, signed receipt, and feedback boundaries
• added a detailed trustworthy retrieval capability flow
• added a detailed receipt-bound feedback shadow-loop flow
• embedded the detailed diagrams in the v0.25.1 announcement
• registered the new and revised assets in visual-claims.json

Why

The previous overview still emphasized the older runtime shape and did not visually explain the v0.25.1 capability contract, recall receipts, or append-only shadow feedback. The new diagrams distinguish durable authority from derived indexes and make the current non-goals explicit: no hash-as-semantic claim, no recall-time vector writes, no active feedback reranking, and no automatic policy path.

Validation

• parsed all three SVG files as XML
• rendered all three SVG files with Inkscape and visually inspected them for clipping, overlap, and crossed labels
• validated visual-claims.json as JSON
• compared the branch against v0.25.1: five documentation/visual files only

No runtime code was changed.