Rewrite repo layout and add latency-driven auto-level flight stack

[jgoppert]

Summary

This is a significant rewrite that reorganizes the
repository around the rdd2/
platform and introduces the new latency-driven
flight-control stack.

For the migration it might be better to not merge these histories and keep the previous repo layout as a historical branch.

Migration of existing boards and platforms is not complete. There is no safety switch, be careful if you fly this, switch A is arming (all the way down to arm, middle/up disarm), switch b up is acro, switch B middle is auto level

The change imports generated estimator and PID
code, adds ACRO and AUTO_LEVEL
flight modes in the hot path, moves the project to
an rdd2-scoped platform
layout, and stages native-sim SITL around
FlatBuffer-based topics and
driver-layer simulation.

What changed

• move the active vehicle/platform implementation
  under rdd2/ with its own
  CMakeLists.txt, Kconfig, README, spec/,
  and west.yml
• add the new hot-path flight stack:
  attitude estimation, rate control, attitude
  control, flight modes, RC input,
  control I/O, motor output, and shell/debug
  support
• import generated estimator and PID code under
  rdd2/src/generated
• add native-sim SITL transport plus simulated
  IMU, RC, and DSHOT drivers
• move DSHOT-related bindings and driver ownership
  under rdd2/
• bump the FlatBuffer message/schema usage to
  carry flight-mode and attitude
  fields

Breaking changes / removals

• remove the old top-level app-centric layout,
  including legacy app/*
  applications and the previous app/rdd2 path
• remove the old shared top-level support layout
  such as lib/core,
  legacy tests, and outdated helper scripts that
  no longer fit the new
  structure
• remove the root west.yml; the manifest now
  lives at rdd2/west.yml
• narrow the repo around the rdd2 vehicle/
  platform path and its associated
  specifications

Review guidance

• review this as a rewrite/migration, not an
  incremental refactor
• start with rdd2/spec/ and rdd2/README.md for
  intended architecture and
  scope
• then review rdd2/src/ for control-path
  behavior and rdd2/drivers/ plus
  rdd2/subsys/sitl/ for integration and
  simulation changes