RFC-ATF-13: Agent Trust Fabric -- Agent Capability Emergence Engine (ACEE). Closing the Multi-Agent Capability Gap with Capability Cluster Detection, Necessity Proof Protocol, and PQC-Signed Capability Witness Records

RFC-ATF-13 specifies the Agent Capability Emergence Engine (ACEE) -- the thirteenth RFC in the ATF Open Standard series published by OMNIX QUANTUM LTD. RFC-ATF-13 closes the Multi-Agent Capability Gap (MAC-Gap): the structural blind spot where two tool calls -- each individually ALLOWED under TCGL -- combine to constitute a capability that is CATASTROPHIC in the composed multi-agent context. Prior ATF layers evaluate tool calls in isolation; ACEE evaluates the composition surface. Five core components: CCG (Capability Cluster Graph): A directed graph where nodes are individual tool calls and weighted edges represent semantic composition proximity. CCG operates pre-execution across all agents in the governance session, identifying clusters whose combined path score exceeds severity thresholds. ACEE-INV-001: CCG scan is mandatory before any multi-agent tool call sequence. NPP (Necessity Proof Protocol): For any cluster with severity CONCERNING or above, NPP requires a structured necessity argument demonstrating that the capability composition is required by the declared mandate. path_score in [0.0, 1.0] is a deterministic pure function of CCG edge weights. CWR (Capability Witness Record): A PQC-signed (ML-DSA-65) governance artifact issued per cluster, containing: cluster_id, member tool calls, path_score, severity classification, necessity proof, and PoGR anchor. Offline-verifiable. ACEE-INV-003: CWR is pre-execution and immutable. CCM (Cross-Session Capability Memory): Append-only registry of capability cluster patterns across sessions. ACEE-INV-008: CCM entries are append-only. Enables TEG computation. TEG (Temporal Emergence Gradient): Measures the rate of capability emergence across sessions, detecting capability drift before it becomes CATASTROPHIC. Five severity classes: BENIGN (path_score=0) -- SUSPICIOUS (0.0-0.3) -- CONCERNING (0.3-0.6) -- CRITICAL (0.6-0.9, TCGL DEFERRED_HAR) -- CATASTROPHIC (>=0.9, SYSTEM HALT). ACEE-INV-005: CRITICAL and CATASTROPHIC clusters MUST route through the TCGL gate before any tool in the cluster executes. 12 Emergence Classification Table (ECT) entries define the normative mapping from (tool_category_pair, composition_type) to severity class. ECT is immutable once published in the RFC. 12 new invariants are introduced: ACEE-INV-001 through ACEE-INV-012. Combined with the 272 invariants of RFC-ATF-1 through RFC-ATF-12, the ATF stack reaches 284 formally specified invariants across 36 protocol families. An implementation complying with RFC-ATF-1 through RFC-ATF-13 is designated ATF-ACEE-Compliant -- the highest compliance tier in the ATF stack as of RFC-ATF-13. Persistence schema: 3 new tables -- atf_cwr_records, atf_capability_clusters, atf_ccm_patterns. All append-only. Regulatory alignment: EU AI Act Art. 9 (multi-agent risk documentation), Art. 10 (training data governance for capability detection), Art. 13 (transparency of composed capabilities), Art. 72 (market surveillance via CWR public verification); NIST AI RMF GV-1.1 (multi-agent governance scope); ISO/IEC 42001 Clause 8.4 (AI system design and composition risk). Related ADR: ADR-218. Test suite: 93/93 PASS. Intellectual origin of CCG graph-theoretic formulation: OMNIX QUANTUM LTD R&D team, June 2026.