docs: centralize Markdown developer manual

docs/src/sitl.md

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+# Host SITL
+
+Host SITL is Kaptan's deterministic closed-loop simulation tier. Gazebo sends
+simulated IMU and declared noisy local position/velocity observations over a
+private loopback protocol. The Rust process runs the ESKF, safety supervisor,
+trajectory, controller, and allocator, then returns four virtual motor
+fractions to Gazebo.
+
+It is a pre-bench software gate, not a physical-flight validation.
+
+## Run the closed loop
+
+```sh
+docker compose -f tools/sitl/compose.yaml up -d --build
+tools/sitl/kaptan-sim ready
+tools/sitl/kaptan-sim move 5 3 1 --wait
+tools/sitl/kaptan-sim land 0 0 --wait
+docker compose -f tools/sitl/compose.yaml down
+```
+
+For the reproducible command-path gate, use:
+
+```sh
+tools/sitl/e2e-smoke.sh
+```
+
+The smoke scenario verifies ascent, translation, descent, landing, flat-plane
+proximity, and zero applied motor fractions after landing. Its reference world
+contains only a visual landing pad; the pad has no collision geometry and
+never contributes a controller or truth observation.
+
+Open <https://app.gazebosim.org/visualization>, connect to
+`ws://localhost:9002`, and leave authorization blank to view the world.
+
+## Limits
+
+This model does not validate terrain, vision, flow/range sensing, wind,
+vibration, battery/ESC behavior, sensor latency/jitter, actuator faults, or
+physical touchdown sensing. Add those only as a reviewed sensor/contact model,
+not as a display-only shortcut.