docs: clarify action receipt verification boundary

[carloshvp]

Summary

Refs #66.

• clarifies the action-receipt verification boundary for embodied workflows in the spec
• adds verification-guide taxonomy for accepted receipts, valid rejections, missing required receipts, and invalid receipts
• adds informative action-receipt fixture shapes without changing the TRACE schema

Validation

• .venv/bin/ruff check src tests
• .venv/bin/mypy src/agentrust_trace
• .venv/bin/pytest --cov=agentrust_trace --cov-report=term-missing
• git diff --check

Docs build note:

• .venv/bin/mkdocs build --strict --site-dir /tmp/trace-action-receipts-site-check currently fails before rendering changed pages because the existing MkDocs config uses docs_dir: ., which MkDocs rejects as the parent directory of the config file.

[imran-siddique]

Approving. The new §3.3.2 (informative) and the verification-guide additions are accurate and well-scoped.

• Purely informative and additive: no change to schema/trace-claim.json, and the new examples/action-receipts/ fixtures are explicitly marked as non-Trust-Records not validated by the schema (examples/README.md + examples/action-receipts/README.md), so additionalProperties: false is not at risk.
• The verification boundary is stated correctly and consistently with the existing §3.3.1 trust-boundary paragraph: a signed controller rejection is valid negative evidence, not a TRACE verification failure, while a signed acceptance does not prove physical completion or functional-safety certification unless a stricter profile defines and trusts that external claim. The four outcomes (receipt_valid_accepted / receipt_valid_rejected / receipt_missing_required / receipt_invalid) are coherent and match between spec, verification.md, and the fixture README.
• Correctly keeps the action-receipt requirement axis independent of build-provenance depth, and the verifier steps (recompute digest, trusted issuer key not the embedded key, chain ordering, session/call binding, report missing/stale separately) mirror the §3.3.1 rules.

CI green. LGTM.