docs: add evidence boundary guide

docs/evidence/EVIDENCE_BOUNDARY_GUIDE.md

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+# Evidence Boundary Guide
+
+OpenAMP Foundry should state what each evidence artifact can and cannot support.
+
+This guide prevents a dry-lab result from being read as stronger than its review level.
+
+## Boundary note
+
+```markdown
+## Evidence boundary
+
+**Artifact:** Evidence certificate, benchmark report, shortlist, review packet, or model note.
+
+**Supports:** What the artifact can support.
+
+**Does not support:** What must not be inferred from it.
+
+**Required next review:** maintainer | evidence | quantitative | external.
+
+**Overread risk:** The most likely way someone could overread this artifact.
+
+**Status:** exploratory | reviewable | bounded | handoff-ready | returned evidence.
+```
+
+## Reviewer guidance
+
+Reviewers should ask for this note whenever an artifact could be read as stronger than it is.
+
+## Contributor guidance
+
+Contributors should write the boundary before writing the headline.
+
+## Acceptance test
+
+A boundary note is adequate when a reader can tell exactly what the artifact supports and what it does not support.