feat: add isolated PoCI multigraph witness quorum

[safal207]

What changed

Adds the seventh PoCI slice: process-isolated multi-graph witness statements and an exact-root quorum coordinator.

The implementation adds:

• a deterministic witness-statement profile;
• three separately spawned witness processes in the canonical demo;
• domain-separated commitments to the frozen source, six graph roots, transition cells, and multi-graph root;
• a 2-of-3 exact-root quorum;
• one-vote-per-witness accounting;
• visible minority dissent without granting extra vote weight;
• replay, nonce substitution, source mismatch, statement tampering, duplicate operator, incomplete graph coverage, and equivocation detection;
• 11 focused unit/integration tests;
• required CI generation of three portable statement files and witness-quorum-report.json.

Reviewer command

python3 scripts/verify_poci_multigraph_quorum.py \
  examples/poci-witness/multigraph/witnesses.json \
  --pretty \
  --statements-dir /tmp/poci-witness-statements

Expected result:

decision: ACCEPT
configured_witness_count: 3
quorum: 2
agreeing_witnesses: 3
consensus graph roots: 6

Byzantine/dissent behavior

Two exact matching root vectors can reach quorum despite one different operator statement. The dissenting witness remains explicit in the report. If no exact vector reaches quorum, the result is BLOCK. Conflicting statements from the same witness in one round are CHALLENGE equivocation.

Trust boundary

This PR demonstrates separate operating-system processes inside one CI runner. statement_root is a tamper-evident hash commitment, not an identity signature. Distinct operator_id values are declared and checked but are not externally proven to represent different organizations or machines.

Stacked PR

Base: agent/poci-multigraph-transition-space-v0.1 / #198

Review and merge after #198, then retarget to main if GitHub does not do so automatically.

Backlog

Epic: #180

Advances the network trigger in #191 by demonstrating interoperating witness processes and a portable quorum report, while keeping discovery, external identity, signatures, and multi-machine networking out of ProofPath core for now.

[safal207]

Validation complete

Final head: 2558c00ae54e28fc6024cffd1590f9e49f50fff2

Both workflows passed:

• PoCI run Add showcase transcript and launch post #12 — Python/Rust conformance, six-graph transition report, all 11 witness tests, three isolated witness subprocesses, exact 2-of-3 quorum, reviewer demo, and artifact upload succeeded.
• Rust run #279 — formatting, clippy with -D warnings, workspace tests, Compute Witness, action-boundary metrics, reusable action, Personal Agent Guard, and Agent Payment Guard all succeeded.

The canonical quorum reached ACCEPT with three agreeing witnesses. Every statement commits the same six graph roots, transition-cell root, source digest, and multi-graph root.

Uploaded artifact:

• poci-conformance-30612219110
• artifact id: 8785910302
• size: 13277 bytes
• SHA-256: e4e43fdd51a642e338e93689a2a3e5bb2592866263e1e56183bf8e7265322385
• expires: 2026-08-14T07:15:54Z

The artifact includes witness-quorum-report.json and the three portable witness statement files.

Trust boundary remains explicit: process isolation is proven inside one CI runner; statement roots are hash commitments, not operator identity signatures.

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