docs: centralize Markdown developer manual

docs/src/reference.md

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+# Reference
+
+## Build-time configuration
+
+Board configuration is immutable in the compiled image. Runtime parameter
+updates and ground-station tuning are not exposed by the checked-in firmware.
+
+`TODO(configuration)`: a future configuration system must validate and stage
+an entire profile while disarmed, commit it atomically, and prove that outputs
+remain inhibited throughout staging, activation, and power-loss recovery.
+
+## Simulation package reproducibility
+
+The host SITL Docker build uses digest-pinned base images and the locked Rust
+dependency graph. The signed Gazebo APT feed is not snapshot-pinned.
+
+`TODO(reproducibility)`: snapshot/version-pin the Gazebo package set and
+verify the repository signing-key fingerprint before treating the simulator
+image as byte-for-byte reproducible release input.