Add architecture and invariants documentation sections

Created new documentation files for the architecture and invariants, detailing the identity, API, build, streaming, and data lineage components, as well as breaking-change rules and compatibility policies. This enhances the overall clarity and structure of the documentation.

Author: stephenway

docs/architecture/index.md

@@ -0,0 +1,15 @@
+---
+title: Architecture
+description: Identity, API, build, streaming, data lineage
+---
+
+# Architecture
+
+| Document | Description |
+|----------|-------------|
+| [Identity Layer](identity-layer) | Edition identity, fingerprinting, invariants |
+| [API Layer](api-layer) | Streaming endpoint, playback prediction, prewarm |
+| [Build Layer](build-layer) | Artifact production, concurrency, determinism |
+| [Streaming Model](streaming-model) | Playback philosophy, device capability |
+| [Data Lineage](data-lineage) | Data flow, entity relationships |
+| [Data Contracts](data-contracts) | Fingerprint invariants, data domains |

docs/invariants/index.md

@@ -0,0 +1,11 @@
+---
+title: Invariants
+description: Breaking-change rules, compatibility policy
+---
+
+# Invariants & Governance
+
+| Document | Description |
+|----------|-------------|
+| [Breaking Changes](breaking-changes) | Version bump, migration, coordination |
+| [Compatibility Policy](compatibility-policy) | Identity, API, playback compatibility |