feat(diagnostics): add health engine

docs/Diagnostics.md

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+# Marketplace Diagnostics & Health Engine - Sprint 14
+
+## Scope
+
+Sprint 14 adds a development-only diagnostics feature for marketplace configuration and runtime health.
+
+Constraints respected:
+- No runtime behavior changes
+- No business logic changes
+- No authentication changes
+- No payment changes
+
+## Architecture
+
+```text
+src/app/features/diagnostics/
+  components/
+    diagnostics-page.component.*
+  services/
+    diagnostics-logger.service.ts
+  models/
+    diagnostics.model.ts
+  validators/
+    bootstrap-diagnostics.validator.ts
+    runtime-diagnostics.validator.ts
+    diagnostics-health-score.util.ts
+  facade/
+    diagnostics.facade.ts
+```
+
+## Health Checks
+
+Current checks cover:
+- bootstrap loaded
+- tenant resolved
+- runtime initialized
+- theme loaded
+- widget manifest loaded
+- section engine reachable
+- configuration engine initialized
+- translations available
+- required assets health
+
+## Bootstrap Validation
+
+Current validator detects:
+- missing required properties
+- unknown widget types
+- duplicate page/section/widget ids
+- unknown layout values
+- invalid feature flags
+- broken page definitions
+- invalid navigation targets
+- missing translations in navigation label keys
+- missing branding media
+
+## Runtime Validation
+
+Current validator detects:
+- missing datasource declarations for widgets that support them
+- widget rendering failures from runtime diagnostics stream
+- broken route expectations
+- configuration fallback usage in local development
+- failed image loading where DOM can observe it
+- missing optional data
+
+## Severity Model
+
+Each entry includes:
+- code
+- severity
+- title
+- description
+- affected component
+- suggested resolution
+
+Levels:
+- info
+- warning
+- error
+- critical
+
+## Health Score
+
+Simple weighted score:
+- critical: -15
+- error: -8
+- warning: -3
+- info: 0
+
+Minimum 0, maximum 100.
+
+## Developer Page
+
+Development-only route:
+- `/__diagnostics`
+
+Page shows:
+- summary
+- health score
+- passed checks
+- warnings/errors/critical counts
+- detailed diagnostics entries
+
+## Logging Abstraction
+
+`DiagnosticsLoggerService` stores diagnostic entries in memory.
+
+`RuntimeDiagnosticsService` now keeps unknown widget events in memory for diagnostics consumption only.
+
+No external logging integration in this sprint.
+
+## Extension Points
+
+Future additions can plug into:
+- new validators under `validators/`
+- new runtime event collectors
+- remote diagnostics export service
+- monitoring integrations (Sentry, Datadog, Grafana, OpenTelemetry)
+- asset/network checks
+- widget render timing checks
+
+## Performance
+
+Production route excluded.
+
+Diagnostics work runs only on diagnostics page access in development mode.