feat(ReferenceFrame): velocity of a trajectory in a frame via derivVec

[zhikaip]

Follow-up to #1592: reads the velocity of a point trajectory off a frame's axes as a
frame.Vector, using derivVec.

• Moves ReferenceFrame.lean into ReferenceFrame/Basic.lean so the folder can grow.
• Adds ReferenceFrame/Velocity.lean with velocityComponents, its vanishing for a
  trajectory at rest, and the identification of IsInertial.velocity with the time
  derivative of the origin.
• Adds the spec lemma IsInertial.origin_vsub to Basic.lean and drops a transitive
  import.
• Records the observer-measured velocity (derivative of the coordinate components) as a
  TODO; it needs regularity of frame.basis in general.
• Updates the API map: one new done entry, the origin-velocity requirement marked done, the
  reference-points half split out and left open.

🤖 Generated with Claude Code

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[zhikaip]

was trying to find an application for derivVec, but I overestimated claude's capabilities to come up with something coherent. To design this properly will require more thought so closing this for now. Apologies for noise