Add synthetic Context Inbox capture proof

[DanielJD1216]

Summary

• add a synthetic Context Inbox capture transition routed through the existing MCP-to-API policy boundary
• retain only owner/namespace-bound provenance, digests, citation metadata, and freshness, never submitted source bodies
• add source-only Context Inbox search with strict state validation, scoped freshness, and no trusted-memory promotion
• enforce immutable append-only state, exact idempotent replay, bounded replay conflicts, and authorization before untrusted-state inspection
• add focused synthetic fixtures, smoke coverage, exports, and documentation

Security and state behavior

• owner authority comes only from authenticated caller context
• namespace and capability policy runs before capture envelope validation
• denied capture and search requests do not inspect caller-supplied state
• capture denials before authorization and owner/namespace state validation return a fresh empty state, including when given module-trusted foreign state
• caller-supplied empty state, callers, requests, nested citations, and capture envelopes are snapshotted once from plain own enumerable data properties, then exact-key validated
• non-empty state is accepted only when created by this in-process synthetic module and tracked through its private trust set; externally supplied non-empty state fails closed
• the full envelope digest binds retained envelope identity, provenance, content digest, and capture freshness, while source-revision identity remains separate for legitimate re-observation
• hidden, inherited, symbol, accessor-driven, own __proto__, transparent or revoked Proxy, sparse-array, and custom-array input fails closed
• capture rejects state containing another owner or namespace rather than returning foreign state
• module-created retained state and capture/search response metadata are deeply frozen
• duplicate request, envelope, and source-revision identities are rejected
• all replay-key conflicts are evaluated before idempotent replay
• capture, replay, and search freshness is owner/namespace scoped and monotonic
• receipts, denials, retained state, and search output exclude submitted source content
• capture and search cannot create pending candidates or trusted memory

Validation

• npm run build && node examples/runtime-skeleton/context-inbox-smoke.mjs
• npm run runtime:skeleton-smoke
• npm run typecheck
• npm test
• npm run runtime:api-policy-smoke
• npm run runtime:mcp-adapter-smoke
• npm run runtime:owner-hosted-smoke
• npm run runtime:gate-b-memory-only
• npm run docs:links
• npm run docs:anchors
• npm run safety:scan
• npm run claims:scan
• npm run readiness:report
• git diff --check

All listed local gates passed on the final reviewed tree. The Gate B memory-only suite passed 46/46 tests. Safety and claims scans reported zero findings.

Scope limits

This remains a public-safe synthetic state-transition proof. It does not add or modify Alpha runtime behavior, migrations, databases, connectors, deployment, real-data handling, package publishing, MCP tool profiles, or trusted-memory promotion.

Closes #298