feat(examples): mcp gateway records with policy decisions and content digests

[jithinraj]

Summary

Adds a runnable example and an outcome-led recipe for the gateway sign-point: an MCP gateway that mediates tool calls issues a portable signed record per call, and a counterparty verifies it offline with the issuer public key. No protocol surface changes; existing types, extensions, and carrier machinery only.

Scope

• New example examples/mcp-gateway-receipts/ (private workspace package, never published): signed tool-definition manifest record (org.peacprotocol/provenance-record), per-call records (org.peacprotocol/access-decision) with the registered access and correlation extensions, content digests (sha256 over a deterministic serialization of input and redacted result), deny records, top-level _meta carrier, offline verification, and two tamper checks (content-binding digest mismatch with a still-valid signature, and an invalid Ed25519 signature).
• New recipe docs/SOLUTIONS/mcp-gateway-receipts.md: what to sign, how it is carried, the five offline verification checks, deny records, registered vs unregistered extension groups, and composition boundaries.
• Runtime smoke test tests/tooling/mcp-gateway-receipts-example.test.ts: imports the example's exported runGatewayDemo() in-process (no build, no network) and asserts the verification outcomes, the deny record, the unknown-extension warning, and both tamper detections.
• Index rows in docs/SOLUTIONS/README.md and examples/README.md.
• docs/releases/facts.json build_targets 104 -> 105 (the new example adds one turbo build target; CI machine-validates this metric against turbo run build --dry).

Explicitly not changed: wire format, schemas, registries, signing, CLI, server behavior, published packages.

Notes

• Digests use a small deterministic stableStringify (recursive key sort) so an independent verifier recomputes the same bytes; the recipe notes a production profile should pin a canonicalization rule such as RFC 8785 JCS.
• org.peacprotocol/access and org.peacprotocol/correlation are registered extension groups; org.peacprotocol/mcp and com.example/gateway are well-formed but unregistered and surface an informational unknown_extension_preserved warning (documented as such).
• The example demonstrates that a valid signature does not extend to unbound content: modifying the delivered result keeps the signature valid but fails the bound result_sha256 check.

Validation

• pnpm demo, pnpm demo:tamper, pnpm demo:show-record all green from a clean install.
• pnpm vitest run tests/tooling: 955 passed (37 files).
• node scripts/verify-release.mjs: all checks pass; build_targets matches turbo dry-run: 105.
• Example build and typecheck pass; prettier clean; git diff --check clean.