Physics-level spring coupling

A clear and concise description of what the feature is

Let springs couple at the physics level, not just by target value. A spring's target can already follow another spring through the FluidValue observer model (useTrail is built on it), but its physics are fixed — config is SpringConfig | ((key) => SpringConfig), resolved once per update via mergeConfig (packages/core/src/types/props.ts:256). This makes the physics reactive: a spring's config (tension/friction/mass) can depend on another spring's live value.

const { open } = useSpring(() => ({ open: 0 }))
const { panel } = useSpring(() => ({
  panel: open,                                       // target-following — ships today
  config: { tension: open.to(o => 100 + o * 200) },  // reactive physics — new
}))
// `open.to(...)` is already a FluidValue; what's new is `config` accepting one.

Why should this feature be included?

Value-level following already ships, so it isn't a differentiator. Physics-level coupling is — it's only possible because react-spring runs a real simulation; there's nothing to couple in a keyframe engine. It unlocks effects you can't express today: stiffness that reacts to a neighbour's position, drag that stiffens its followers, and mutually-reacting springs.

Scope / non-goals

• Value-level target following already ships (FluidValue, useTrail) — not in scope.
• Core work: widen config to accept reactive/fluid inputs. This is additive — existing static configs are unaffected, so it isn't a breaking change.
• Extensions (separate, and the only breaking part): coupling on another spring's velocity would mean exposing SpringValue.velocity reactively (today it's a number snapshot), and bidirectional coupling needs cycle handling in the observer graph.

Start in packages/core/src/types/props.ts at the config definition and trace how mergeConfig resolves configuration during updates. Read the FluidValue observer model used by useTrail. Done means config can accept reactive or fluid inputs for spring physics while existing static configurations remain unaffected.