M1: Flag-gated log query compatibility aliases

[hshimizu]

Context

Flag-gated compatibility aliases (api.md §8.1): the five M1 paths /loki/api/v1/{query_range, query, labels, label/{name}/values, series} bound to the native handlers.

Scope

• Alias mounting under PULSUS_COMPAT_ENDPOINTS=true only (404 otherwise)
• Response-shape verification against the third-party datasource's expectations
• e2e: Grafana log datasource pointed at the compat surface renders queries from the M1 subset

Acceptance criteria

• Aliases 404 when the flag is off; identical results to native paths when on
• Datasource smoke test documented (manual or automated) and passing