Define derived rail and Qonto readiness architecture

docs/adr/ADR-0005-derived-rail-composition.md

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+# ADR-0005: Derived Rail Composition
+
+Date: 2026-07-26
+Status: Accepted
+
+## Context
+
+Knative provides distinct workload activation and revision semantics but runs
+on Kubernetes. Treating it as an unrelated peer rail would duplicate generic
+workload lifecycle and substrate assumptions.
+
+## Decision
+
+Railiance distinguishes base and derived rails.
+
+`rail-kubernetes` is the default base rail for platform workloads.
+`rail-knative` is a derived rail that inherits a versioned
+`rail-kubernetes` common contract and overrides only Knative-specific
+activation, revision, autoscaling, traffic, cold-start, and rollback semantics.
+
+The detailed contract is
+[`docs/rail-composition-contract.md`](../rail-composition-contract.md).
+
+## Consequences
+
+- New platform workloads continue to use `rail-kubernetes` unless a specialized
+  rail is justified.
+- Derived rails declare base-rail compatibility rather than copying lifecycle
+  contracts.
+- Fabric and conformance tooling must understand rail dependency and readiness.
+- Knative installation stays with the S2 substrate owner.