docs(ri-reports): align the README index with the refined report shape

[pepebndc]

Goal

The four RI reports were restructured to the shape Curtis asked us to template from the DEX doc (RI1 on main via #28/#33/#34/#35; RIs 2-4 in #38, #37, #36). This PR updates the index README, which still described the old report layout and was already stale against 01-dex on main.

Changes

• Settlement spine bullet: atomic DvP is batch settlement via SettlementFactory_SettleBatch, or SettlementFactory_SettleBatchWithAttestation when the factory requires D1 attestation (the path the reports build on).
• Decided rails (D1-D4): D1 is party-applied attestation through SettleBatchWithAttestation + the factory admin's TrustedAttesterRegistry (the node-applied D1ComplianceHook framing is gone from the reports); D3 wording is single-synchronizer; D4 is per-role privilege transfer with value-moving roles envisioned as N-of-M (the single-admin framing is outdated).
• Composition table: section references updated to the new numbering (composability now lives in each report's section 6); dropped the "DEX cites PriceOracle for future stable pools" row, since the DEX report now lists external oracles as out of scope.
• Cross-synchronizer model (canonical) section removed: no report carries the section-8 [FUTURE] chapter anymore. Replaced with a short scope note (out of scope for M1, single-synchronizer; cross-chain remains the Stablecoin RI's product).
• Implementation status: the per-report "Implementation Status (Code Map)" tables no longer exist; the table now links each report's section 2 "Core Components and Library Mapping" anchor. Spine test count refreshed 33 -> 54.
• Report conventions: section order updated to the six-section shape.

scripts/refresh-ri-anchors.sh: 376 OK, 0 drift, 0 error.

Merge order

Merge after #36, #37 and #38: the #core-components-and-library-mapping anchors for reports 02-04 only exist on those branches (the 01-dex anchor is already valid on main).

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