docs(decisions): ADR 0002 — tier hierarchy

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 # ADR-0002 · Tier hierarchy: Router / Cell / Core / Hub
 **Status**: accepted
 **Date**: 2026-05
 ## Context
 The platform is distributed across multiple physical components. Without a clear naming convention, conversations devolved into ambiguous terms ("the box", "the gateway", "the brain"). We needed a vocabulary that:
 - Reflects the actual physical/logical separation.
 - Scales from minimal deployments (one site, few cameras) to large fleets.
 - Is brandable and can be used in marketing without sounding generic.
 ## Decision
 Four tiers, named:
 | Tier | Name | Hardware | Role |
 |---|---|---|---|
 | 0 | **Blocao Router** | OpenWrt-class device (e.g. GL.iNet GL-MT6000) | Gateway, broker, console host, GitOps reconciler |
 | 1 | **Blocao Cell** | RK3588 SBC (e.g. Banana Pi BPI-W3) | Edge AI inference, recording, forensic API |
 | 2 | **Blocao Core** | Jetson Orin NX or similar | Multi-Cell coordinator, heavier models. **Optional** |
 | 3 | **Blocao Hub** | EU sovereign bare-metal (Hetzner Falkenstein/Helsinki) | Multi-site control plane, fleet management |
 **Topology notation**: `R+1` (router + 1 Cell), `R+3+C` (router + 3 Cells + 1 Core), `R+0` (router only, no edge AI yet).
 Sites are identified as `BL-<slug>` (e.g. `BL-LAB`, `BL-WAREHOUSE-N`).
 ## Consequences
 **Good**:
 - Each name is short, brandable, and used as-is in product literature.
 - "Router + Cell + Core" follows a biological metaphor (smallest → larger units of integration) that customers grasp quickly.
 - Hub is clearly remote, separating it from on-site components.
 **Bad / trade-offs**:
 - "Cell" can be confused with "cellular" (mobile networks). We accept this — context disambiguates, and "blocao cell" + "LTE failover" don't appear together often enough to confuse.
 - Customers occasionally call the Cell "the AI box". We don't correct it in early conversations; the official term sticks once they read docs.
 ## Alternatives considered
 - **Edge / Gateway / Coordinator / Cloud**: too generic, can't be branded.
 - **Node / Hub / Core / Cloud**: "node" is overused, "cloud" we explicitly reject.
 - **Brain / Spine / Eye / Memory** (biological): too cute for industrial buyers.