feat(fanout): variadic composition + fanout(keypaths:into:)

[luizmb]

What

Two point-free ways to fan a value out over several accessors and feed the pieces straight into a multi-argument function (typically a memberwise init): a variadic >>>/<<< overload, and fanout(keypaths:into:).

Why

Narrowing a large value into a smaller one whose init takes the parts as separate arguments — e.g. deriving a feature's Environment from a big World — is a constant shape. Plain fanout(…) >>> Env.init can't type-check because Swift (SE-0110) treats a tuple argument and a multi-argument parameter list as distinct types. A variadic overload of >>> bridges the two; for callers who'd rather avoid the operator, fanout(keypaths:into:) closes the loop in one call.

struct Env: Sendable { init(badge: Int, save: Int) { … } }

let a: @Sendable (World) -> Env = fanout(\.badge, \.save) >>> Env.init
let b: @Sendable (World) -> Env = fanout(keypaths: \.badge, \.save, into: Env.init)

The variadic >>> coexists with the single-argument overload without ambiguity (single-output → plain overload, tuple-output → variadic).

Changes

• CoreFPOperators/Function/FunctionComposition.swift — add variadic >>>/<<< overloads composing (Root) -> (repeat each T) with (repeat each T) -> Out.
• CoreFP/Utilities/Fanout.swift — add fanout(keypaths:into:) (key paths constrained to Sendable); make the existing fanout(_:) return @Sendable.
• Tests — FanoutTests (new) + variadic composition cases in FunctionCompositionTests.
• Docs — README fan-out subsection; DocC PointFreeStyle + CoreFPOperators articles.

Full suite green (2775 tests); swiftlint --strict clean on changed files.

🤖 Generated with Claude Code