Add enterprise docs structure and include pending core maintenance updates.

docs/architecture.md

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+# Architecture
+
+## System Context
+
+CBDD is an embedded database library consumed in-process by .NET applications. The host application owns process lifecycle, filesystem placement, and operational policy.
+
+External dependencies:
+- .NET runtime and SDK
+- Local filesystem
+- Optional CI and package registry for build/release
+
+## Containers And Major Components
+
+1. `CBDD.Core`
+- Owns storage engine, transaction protocol, WAL, indexing, query planning, and CDC plumbing.
+
+2. `CBDD.Bson`
+- Owns BSON document model and span-based serialization/deserialization primitives.
+
+3. `CBDD.SourceGenerators`
+- Generates mapping code at compile time for entity serialization and collection initialization.
+
+4. Consumer application
+- Defines entities, `DocumentDbContext` subclasses, and operational behavior.
+
+## Data Flow
+
+1. Consumer invokes collection API through `DocumentDbContext`.
+2. Mapper layer serializes entities to BSON via generated mappers.
+3. Storage engine writes page data and WAL entries.
+4. Index subsystem updates primary and secondary indexes.
+5. Transaction commit persists durable state and emits CDC notifications where enabled.
+6. Query path evaluates expression plans and uses indexes or scan fallback.
+
+## Reliability Model
+
+- Write-ahead logging enforces durability before logical commit completion.
+- Snapshot isolation supports concurrent reads with transactional correctness.
+- Recovery logic replays WAL on restart to restore committed state.
+
+## Cross References
+
+- Operational procedures: [`runbook.md`](runbook.md)
+- Deployment and rollback: [`deployment.md`](deployment.md)
+- Security controls: [`security.md`](security.md)
+- Detailed protocol reference: [`../RFC.md`](../RFC.md)