feat: add Proof of Causal Integrity v0.1 contract

[safal207]

What changed

Adds the first normative contract slice for Proof of Causal Integrity (PoCI) v0.1:

• protocol actors, invariants, lifecycle, and decision semantics;
• stable ACCEPT / HOLD / BLOCK / CHALLENGE model;
• stable reason-code registry and fail-closed precedence;
• strict Draft 2020-12 Action Proof Envelope JSON Schema;
• canonical JSON, domain separation, and envelope-root rules;
• first schema-valid bounded compute-purchase envelope;
• reviewer-friendly local schema-validation quickstart.

Why

ProofPath already verifies high-risk actions at the execution boundary. This PR defines a portable evidence contract that can later be implemented independently by Python, Rust, and external witnesses without collapsing CML, LTP/T-Trace, LiminalDB, or Ibex into one repository.

Validation

• Draft 2020-12 schema structure checked with jsonschema.
• examples/poci-witness/action-proof-envelope.valid.json validates with zero schema errors.
• Diff contains five new contract/example files only.

Current limitations

• Placeholder artifact digests are intentional.
• envelope_root is null until the reference canonicalizer and golden vectors are implemented.
• This PR does not claim distributed consensus, witness independence, token economics, hardware truth, zkML correctness, or real-world truth.

Backlog links

Epic: #180

Advances:

• P0: Specify PoCI v0.1 protocol and decision semantics #181
• P0: Define Action Proof Envelope v0.1 JSON Schema #182
• P0: Specify deterministic canonicalization and evidence hashing #183

Next PR:

• fixture corpus and threat-to-fixture mapping (P0: Add PoCI reference and adversarial fixture corpus #184, P0: Define PoCI threat model and challenge taxonomy #189)

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