docs: document legacy session v1 interop and duplicate-vs-BadMac ranking

[jlucaso1]

Summary

Documents whatsapp-rust#1072 "feat(signal): add typed legacy session interop" in advanced/signal-protocol.mdx:

• New Legacy session v1 interop subsection under "Record components" — the opt-in legacy-session-interop Cargo feature, the typed decoded-v1 model (LegacySessionRecordV1, LegacySessionV1, etc.), the import path (LegacySessionRecordV1::from_indexed_sessions → into_session_record), the operational (non-byte-exact) export path (SessionRecord::into_legacy_session_v1_operational), the previousCounter = -1 floor, and the typed rejection errors for non-representable state.
• Extended Duplicate message detection (under Security Considerations) to cover the decrypt-path behavior that ships unconditionally (not feature-gated): a DuplicatedMessage from one session no longer aborts the search — it continues into archived sessions and closed receiver chains — and a recognized duplicate now outranks BadMac in the final error classification, so a replay is acknowledged instead of triggering a spurious retry receipt.

Test plan

• Read the merged PR's diff, new module (wacore/libsignal/src/protocol/legacy_session.rs), and Cargo.toml feature wiring to verify the described public API and behavior
• Docs site preview (Mintlify) renders the new section and anchors correctly

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Generated by Claude Code

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Summary by cubic

Documents the new opt-in legacy-session-interop for typed import/export between decoded legacy SessionRecord v1 and the canonical record, and clarifies duplicate message handling in the decrypt path. Also covers continued search into archived/closed chains and ranking DuplicatedMessage above BadMac.

• New Features
  • Legacy session v1 interop: behind legacy-session-interop in wacore-libsignal; import via LegacySessionRecordV1::from_indexed_sessions → into_session_record; export via SessionRecord::into_legacy_session_v1_operational; floors previous_counter = -1 to 0; rejects non-representable state with typed errors; secrets redacted in Debug.
  • Duplicate detection: a DuplicatedMessage no longer ends the search; archived sessions and closed receiver chains are tried; final error prefers DuplicatedMessage over BadMac to avoid spurious retry receipts.

^{Written for commit 4d10589. Summary will update on new commits.}

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Reviewing files that changed from the base of the PR and between df10074 and 4d10589.

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• advanced/signal-protocol.mdx

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Project	Status	Preview	Updated (UTC)
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[greptile-apps]

Greptile Summary

This PR documents legacy session interoperability and revised duplicate-message handling. The main changes are:

• Adds the feature-gated legacy session v1 import and export APIs.
• Describes migration validation, representability errors, and secret redaction.
• Explains continued session search after duplicate detection.
• Documents the final ranking of DuplicatedMessage over BadMac.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed documentation.

Important Files Changed

Filename	Overview
advanced/signal-protocol.mdx	Adds legacy session v1 interoperability documentation and explains duplicate-message search and error ranking.

_{Reviews (1): Last reviewed commit: "docs: document legacy session v1 interop..." | Re-trigger Greptile}