Add real equilibrium (LCFS / flux-surface) plotting to the Sensors view

[matt-pharr]

The Sensors view had a synthetic LCFS/flux-surface overlay (mockEquilibrium)
that produced garbage; it's been removed. The overlay now draws only a real
backend equilibrium node — absent today, so nothing is shown (no plot trace, no
legend entry). The EquilibriumNode contract, the SensorsTab overlay code, the
time cursor, and the "equilibrium" toggle are all still wired — this issue is just
to supply the real data.

Options to source it from MDS (per shot, per timestep)

• Full EFIT: fetch per-timestep EFIT ψ_N(R,Z) + boundary and serve as the
  equilibrium node (richest; heaviest).
• Shape channels (lighter): DIII-D has channels for major/minor radius,
  elongation, triangularity (upper/lower), squareness, etc., and possibly the LCFS
  boundary itself. Reconstruct an approximate boundary from these for the overlay.

Wiring already in place

• EquilibriumNode kind in contract.ts.
• SensorsTab renders equilibrium.{r,z,psi_n,boundary,axis,levels} + time slider.
• Just needs a real equilibrium builder in the service (time-parametrized).