[1ptqc-KD-97bsBG-pVJG]

Closes #24.

Summary

This PR completes validation of the generic external simulator bridge against Basilisk while preserving ASTRO's existing ROS-facing contract.

It keeps the bridge simulator-agnostic, isolates Basilisk-specific behavior behind an internal backend, and adds both deterministic regression coverage and pinned-runtime Basilisk validation.

What changed

• added the external_sim_bridge ROS 2 package as a generic bridge between ASTRO and external simulators
• preserved the existing public ROS contract:
  • env_data remains a 6-element Float64MultiArray ordered [x, y, z, vx, vy, vz]
  • actuation_cmd remains distributed_satellite_sim/srv/ActuationCmd
• kept gnc_node unchanged
• added a deterministic linear/fake backend for bridge-core regression coverage
• added a Basilisk backend for the fixed translational validation scenario
• documented the bridge contract, Basilisk assumptions, and manuscript/paper alignment in src/external_sim_bridge/doc/design_note.md
• re-anchored the bridge regression test to the canonical DLQR fixture owned by distributed_satellite_sim
• installed the canonical trajectory fixture from distributed_satellite_sim so downstream tests can resolve it through ament_index
• added a dedicated GitHub Actions workflow that validates the bridge in the ROS Kilted image with bsk==2.10.0

Basilisk validation details

• pins the validation runtime to bsk==2.10.0
• validates direct Basilisk backend behavior
• validates the end-to-end gnc_node -> actuation_cmd -> bridge -> Basilisk loop
• fixes Basilisk command handling so post-start actuation updates are written with the current sim timestamp rather than the default 0 ns

Why this matters

This PR moves the branch from “generic bridge implemented” to “generic bridge validated against Basilisk” without turning the bridge into a Basilisk-only design.

The simulator-specific logic remains in the Basilisk backend. The ROS-side contract remains unchanged and reusable for additional external simulators later.

Testing

Validated in ghcr.io/accommodus/astro:latest with ROS 2 Kilted and bsk==2.10.0:

• rosdep install --from-paths src --ignore-src -y
• colcon build --packages-select distributed_satellite_sim external_sim_bridge
• ASTRO_EXPECT_BASILISK=1 colcon test --packages-select external_sim_bridge
• colcon test-result --verbose

Result:

• 11 tests, 0 errors, 0 failures, 0 skipped

Notes for reviewers

• the validation scenario is intentionally narrow: a fixed translational Basilisk path used to confirm bridge compatibility
• this PR does not broaden the public ROS interface or implement the richer ROS-side GNC decomposition discussed elsewhere
• the bridge remains generic; Basilisk is the first validated backend, not the contract itself