docs: centralize Markdown developer manual

docs/src/architecture.md

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+# Architecture
+
+Kaptan separates immutable board configuration from platform-owned runtime
+resources and portable flight logic.
+
+```text
+board declaration
+        │
+        ▼
+F722 platform / RTIC application ──► data channel ──► MAVLink telemetry
+        │
+        ├── physical sensor drivers
+        └── motor-pin inhibit interlock
+
+portable libraries
+  math ─► navigation ─► control ─► safety
+```
+
+The board contract selects one immutable `BoardProfile`. The checked-in
+profile is `TelemetryOnly`; `ExternalControlOnly` is represented explicitly
+but rejected at validation time because it has no reviewed runtime.
+
+The data channel owns rate selection, mailbox consumption, message encoding,
+and bounded TX classification. Board code supplies physical routes and
+telemetry configuration; it does not receive generic output authority.
+
+## Ownership rules
+
+- A peripheral owner is local to the platform runtime.
+- Portable crates use typed units and validated values rather than raw wire or
+  timer representations.
+- A safety permit is epoch-bound and must be revalidated at the only nonzero
+  output boundary.
+- A failed observation or numerical check does not partially commit estimator,
+  controller, allocator, or safety state.