The "Fixed" designation is vital for this build, as earlier iterations suffered from severe engine-level bugs.
Unity’s physics system runs at a fixed timestep (default 0.02 seconds). FixedUpdate is called exactly once per physics tick, regardless of frame rate. For a malevolent planet affecting Rigidbody2D gravity, this is mandatory.
Day 2 focused on "un-breaking" what was lost during previous iterations. Re-enabling Legacy Scenes:
playerRb.gravityScale += gravityIncreasePerSecond * Time.fixedDeltaTime;