docs: put MinIO first-class on ground, add executive overview

Treat MinIO as existing stack infrastructure (default T3 warehouse, not
on the drone), add a one-page executive map, and record placement as an
open team question rather than a rip-and-replace.

docs/06-environments.md

@@ -12,12 +12,12 @@ graph LR
         SLICE["T4: data slices"]
     end
     subgraph fleet [1 — Fleet, in flight]
-        D1["drone: Compose data plane<br/>T0 hot + T1 NVMe + T2 MinIO"]
+        D1["drone: Compose data plane<br/>T0 hot + T1 NVMe + T2 peer cache"]
         D2["drone …"]
     end
     subgraph ground [2 — Ground, on-prem]
         DOCK["base station docks"]
-        DWH["T3 warehouse<br/>object store + Parquet/DuckDB"]
+        DWH["T3 warehouse<br/>MinIO + Parquet/DuckDB"]
         K3S["k3s: CI runners, registry mirror,<br/>sim farm, dashboards"]
         TRAIN["model training (out of scope)<br/>reads T3, ships weights"]
     end
@@ -43,7 +43,7 @@ The permanent installation. This **is** allowed to be a cluster — links are wi
 
 | Component | Runs on | Role |
 | --- | --- | --- |
-| **Warehouse (T3)** | Object store (MinIO or equivalent) + Parquet | Every flight of every drone; the system's long-term memory |
+| **Warehouse (T3)** | **MinIO** (already in the stack) + Parquet | Every flight of every drone; the system's long-term memory |
 | **Base station docks** | Bare metal | Wired offload + integrity audit + drone provisioning ([03](03-data-platform.md), [05](05-network-security.md)) |
 | **Registry mirror** | k3s | In-air-gap container registry + artifact storage; the only software source drones ever see |
 | **CI runners** | k3s | Multi-arch builds, tests, scans ([11](11-cicd-delivery.md)) |