Add Railiance canary chart template

docs/canary-helm-template.md

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+# Canary Helm Template
+
+Generated Railiance overlays include a stage-aware Helm chart for Stage 2
+canaries and Stage 3 stable promotion.
+
+The chart keeps stable and canary release identities explicit:
+
+- `railiance.stableRelease` names the current stable release;
+- `railiance.canaryRelease` names the Stage 2 candidate release;
+- `railiance.stage` selects the rendered identity, labels, and selectors;
+- `railiance.previousStable` records rollback context before promotion.
+
+## Traffic Shape
+
+The default Stage 2 values use an isolated canary ingress:
+
+```yaml
+railiance:
+  stage: canary
+  traffic:
+    mode: isolated
+
+ingress:
+  enabled: true
+```
+
+This creates canary Deployment, Service, and Ingress resources without changing
+the stable release. For environments that use Traefik weighted routing, set:
+
+```yaml
+railiance:
+  traffic:
+    mode: weighted
+    provider: traefik
+    stableWeight: 95
+    canaryWeight: 5
+```
+
+The chart then renders a `TraefikService` and `IngressRoute` that split traffic
+between the stable and canary services. Other ingress controllers can use the
+same stable/canary values layout with controller-specific annotations or a later
+provider template.
+
+## Observability And Safety
+
+Generated workloads include:
+
+- Prometheus-compatible scrape annotations on pods and services;
+- readiness and liveness HTTP probes;
+- conservative resource requests/limits for single-node clusters;
+- separate `values/stage2-canary.yaml` and `values/stage3-production.yaml` so
+  canary exposure and stable promotion can be reviewed independently.
+
+Run `tests/stage2-template.sh` in the overlay repo before any Stage 2 attempt.
+It verifies the scaffold and runs `helm template` when Helm is available.