Add RandomGenerable and the opt-in @RandomGenerable derivation macro (Derive trait)

[ibrahimkteish]

Summary

Adds generator derivation for user-defined types, gated behind a new opt-in Derive package trait so the swift-syntax build cost is only paid by consumers who want the macro.

• RandomGenerable protocol (always available, no trait needed): types expose a canonical static var generator. Standard types ship pre-conformed with simple, documented defaults — fixed-width integers (full range), Double/Float (0...1), Bool, String (8 alphanumerics), Character (ASCII letter), Optional (nil half the time), Array (0–10 elements).
• @RandomGenerable macro (requires the Derive trait): derives a conformance for structs (through the memberwise initializer) and enums (uniform case selection, associated values included). @Gen(...) overrides the generator for a single stored property. Derived generators are ordinary generators — combinators and seeded RNGs apply.
• With the trait off (the default), swift-syntax is never compiled; the core library remains a zero-build-cost dependency. It is still fetched at resolution time (SE-0450 applies traits after resolution).

@RandomGenerable
struct User {
    @Gen(IntGenerator(in: 18...99)) var age: Int
    var name: String        // 8 random alphanumerics
    var isAdmin: Bool
    var nickname: String?   // nil half the time
}

let team = User.generator.array(5).run()

Consumers opt in from their manifest (Swift 6.1+), or in Xcode 26.4+ via package-trait settings:

.package(url: "https://github.com/ibrahimkteish/SwiftRandomKit.git", from: "2.2.0", traits: ["Derive"])

Implementation notes

• swift-tools-version 6.0 → 6.1 (required for traits). Older toolchains keep resolving 2.1.0; SwiftPM skips manifests it cannot read.
• The macro target and its swift-syntax products hang off condition: .when(traits: ["Derive"]) edges, so nothing macro-related enters a trait-off consumer's build graph.
• Diagnostics for unsupported shapes: classes/actors, generic types, missing type annotations, @Gen on multi-bindings or non-properties, case-less enums.
• Macro expansion tests use MacroTesting inline snapshots in a dedicated SwiftRandomKitMacrosTests target; runtime behavior (determinism, @Gen ranges, enum coverage, nesting, combinators) is covered in the main test target behind #if Derive.
• CI runs the suite in both trait configurations. Trait-enabled test runs pass --disable-experimental-prebuilts: MacroTesting links swift-syntax as a library, which is incompatible with SwiftPM's prebuilt swift-syntax binaries.

Test plan

• swift test (trait off): 187 tests, 38 suites — pass
• swift test --traits Derive --disable-experimental-prebuilts: 208 tests, 40 suites — pass
• swift test -c release: pass
• Example target runs in both configurations (derive section prints only with the trait)
• First CI run validates the Swift 6.1 pin on macos-15 and the prebuilts flag on SwiftPM 6.1

Release intent: 2.2.0 (additive; changelog entry is staged as Unreleased — date/tag on release).