[Ri0n]

Summary

Complete the transport-facing OTRv3 negotiation and control-message layer on top of the routed AKE/Data core.

The existing Psi plugin still uses libotr; this PR extends standalone qca-otr and validates the new native paths against libotr 4.1.1 as an interoperability oracle.

Implemented

• OTR query generation/parsing and version negotiation (?OTRv3? and combined queries)
• OTRv3 whitespace-tag detection/generation for opportunistic policy
• manual/opportunistic/always policy behavior needed by the current plugin
• encrypted envelope plaintext || NUL || TLVs
• strict TLV codec with malformed/truncated input handling
• disconnected TLV and FINISHED-state semantics, including restart
• symmetric-key TLV / extra-key plumbing compatible with libotr
• canonical libotr-compatible protocol error generation/parsing and unreadable/error handling
• per-instance routing across negotiation and control paths
• bidirectional public-API libotr 4.1.1 interoperability coverage for query/whitespace negotiation, generic TLVs, disconnect, SYMKEY and protocol errors
• Qt5 and Qt6 CI coverage with the full oracle suite

Validation

The Stage 2 exit criterion is satisfied: start/disconnect/error and encrypted TLV traffic round-trip with libotr 4.1.1 using the public message API, while qca-otr itself does not depend on libotr message wrappers.

The final HEAD is green on both Qt5 and Qt6. PLAN.md marks Stage 2 completed and README.md documents the implemented negotiation/control surface and oracle coverage.

Qt6 consumes the published QCA 3.0.3 Ubuntu release package; Qt5 remains pinned to the same QCA 3.0.3 source tag for compatibility coverage.