[Grafana] Detect Prometheus recording-rule series with no target field; flag instead of emitting silently-empty panels

## Problem statement

PromQL dashboards frequently reference **Prometheus recording rules**: precomputed series with colon-separated names (e.g. `node_namespace_pod_container:...:sum_rate5m`, `cluster:namespace:pod_cpu:active:...`). These series are materialized by Prometheus at eval time and do not exist in any non-Prometheus target (OTLP, bulk, etc.).

Today the migrator translates them into ordinary field references and marks the panel `migrated` / `migrated_with_warnings`, so on a non-Prometheus target the panel renders empty with no signal that the underlying series is structurally absent. This is distinct from metric-name mapping (#309): a recording rule has **no single target field to rename to**; its value must be recomputed from base metrics or the panel redesigned.

## Proposed change (general capability)

- Detect recording-rule-style identifiers during classification (colon-separated names are the canonical spelling).
- When the target is not a Prometheus-recording-rule-bearing index, either:
- mark the target/panel `not_feasible` (or a dedicated data-readiness state) with a clear reason, and/or
- flag it in `required_target_contract.json` as recording-rule-derived and absent, and/or
- where the base expression is recoverable, emit a redesign task with the base-metric ES|QL equivalent.
- Surface it in the run summary and manifest reasons so an empty panel is not mistaken for a fixable data gap.

Preserve degrade-gracefully; do not hide the semantic gap.

## Source / target impact

Grafana source → any non-Prometheus target. Broadly affects kube-prometheus-stack / kubernetes-mixin and any dashboard built on recording rules.

## Additional context

Classification in `observability_migration/adapters/source/grafana/rules.py` / `promql.py` / `translate.py`. Related: #309, #271, #311.

Start in observability_migration/adapters/source/grafana/rules.py, promql.py, and translate.py, following how PromQL identifiers are classified and translated. Trace the run summary and manifest reason paths. Done means recording-rule-style identifiers targeting non-Prometheus data are visibly flagged as absent or not feasible rather than producing silently empty panels, while graceful degradation remains intact.

Written by the indexing model from the issue text.