docs: add changelog

Author: LalaSkye

CHANGELOG.md

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+# Changelog
+
+This changelog records human-readable evidence changes for `LalaSkye/execution-boundary-lab`.
+
+Git history remains the source of commit-level truth. This file provides a buyer-readable / inspection-readable summary layer.
+
+## 2026-05-11
+
+- Added `RECEIPT.md` to state the repository object, evidence class, failure-harness role, proof surface, and claim boundary.
+- Added `CLAIM_BOUNDARY.md` to separate allowed deterministic-failure claims from forbidden production / compliance / validation claims.
+- Established quiet evidence-custody pattern aligned with the wider execution-boundary repo chain.
+
+Evidence effect:
+
+- Improves inspectability.
+- Makes the negative-case role explicit.
+- Keeps the repository bounded to deterministic failure demonstration and conformance-surface work.
+
+Claim boundary:
+
+- This records documentation hardening only.
+- It does not claim new technical capability, adoption, validation, compliance, production readiness, or production security coverage.