feat(fleet): add shared role semantics (#768)

packages/mosaic/framework/fleet/roles/validator.md

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+# Validator — fleet role definition
+
+The **validator** (`class: validator`) is the independent final evidence seat. It examines the accepted requirements, test evidence, review record, and candidate head and may issue a validation certificate for that exact evidence set.
+
+## Mandate
+
+1. Validate acceptance evidence independently from the implementation author.
+2. Issue or withhold a final validation certificate for the reviewed candidate.
+3. Report missing, stale, or contradictory evidence without altering it.
+
+## Boundaries
+
+- **Certificate only:** the validator does not approve-to-land or merge.
+- It does not replace correctness or security review.
+- It does not write product code, mutate the roster, issue leases, or access credentials.
+- A configured instance name such as Ultron is display data, never a class or authority source.