docs(standards): portable signed records evidence primitive

[jithinraj]

Summary

Adds a neutral, standards-facing reference that defines the portable signed interaction record as an evidence
primitive: a single docs-only file, docs/standards/portable-signed-records-evidence-primitive.md. It states the
record format and its properties as facts drawn from this repository, and points to the crosswalk for the
per-context mapping.

Scope

Docs-only. No example, package, schema, registry, type URI, wire format, signing, public API, or release-state
change; no build_targets change (a docs/standards/*.md file is not a build target). Every relative link
resolves on disk.

What it contains

• A definition section: the frozen receipt wire format, the compact JWS container, Ed25519 (EdDSA) signatures
  with OKP JWK keys, RFC 8785 JCS canonicalization, SHA-256 self-describing digests, and offline verification.
• An explicit non-goals section that reproduces the repository's canonical scope paragraph and boundary
  statement, so the primitive is not mistaken for a governance, policy, payment, identity, orchestration, or
  monitoring layer.
• Properties, carriers (HTTP header, MCP _meta, A2A metadata, with the receipt_ref = sha256(receipt_jws)
  binding), and a composition section that points each context at its existing PEAC example or recipe and at the
  crosswalk.
• A references section citing the relevant RFCs by number and canonical RFC Editor URL, plus version-pinned facts
  from docs/releases/facts.json and specs/kernel/registries.json.

Boundaries

The document is descriptive and neutral. It makes no compliance, certification, endorsement, adoption,
partnership, or conformance claim. Standards and RFCs are cited only as primary references.

Validation

• File is plain ASCII with LF line endings and no hidden, bidirectional, zero-width, or control characters
  (verified).
• All linked artifacts exist; relative links resolve. The crosswalk it points to is on main.
• RFC numbers and titles were re-verified against the RFC Editor before authoring; version-pinned facts were
  re-read from the repository.
• entry-links-doc-truth, public-artifact, and planning-leak checks pass; pnpm lint (0 errors),
  pnpm typecheck:core, doc-truth, and final-hygiene pass; git diff --check clean.