Add construct (polymorphic function builder) to Made.Product

[halotukozak]

Follow-up to #51 (typed-builder).

Made.Product.fromTuple covers the typed builder use case. Magnolia-style construct
may also be valuable for derivation typeclass scenarios where a per-field function
materializes values.

Proposal (shape TBD)

One possible signature — exact shape not decided:

def construct(f: [e <: MadeFieldElem] => (elem: e) => elem.Type): Type

Alternatives worth considering:

• Magnolia-style homogeneous [R](f: MadeFieldElem => R): T with cast
• Rank-2 trait trait Constructor[T] { def apply[X](elem: MadeFieldElem { type Type = X }): X }
• Typeclass-aware variant requiring F[t] summon per field

Implementation notes (if polymorphic-function shape)

• Polymorphic function application inside macro quotes
• Macro-time unrolling of Elems tuple to preserve per-field path-dependent typing at each f($e) call site
• Builds on fromTuple — generated body roughly m.fromProduct((f(e0), f(e1), ...))

Why deferred

Requires polymorphic function application inside quotes plus macro-side tuple unrolling — more
involved than fromTuple. Deferred to keep #51 minimal. Non-breaking to add later.

Concrete consumer use cases would help nail down the exact API shape.