Add Scala 3.7.4 cross-build

[Copilot]

Adds Scala 3.7.4 as a cross-build target alongside the existing 2.12/2.13 builds, eliminating the need for CrossVersion.for3Use2_13 workarounds in downstream projects.

Key challenges

• data-class is Scala 2 only: io.github.alexarchambault::data-class:0.2.7 has no _3 artifact. All files using @data annotations must be handled separately.
• Stream removed in Scala 3: replaced by LazyList.

Approach: version-specific source directories

Source files are split into src/ (version-agnostic), src-2/ (Scala 2.x), and src-3/ (Scala 3):

File	Change for Scala 3
Version.scala	Stream → LazyList, @data class → case class
VersionInterval.scala	@data class → case class
ModuleMatcher.scala	@data class → case class, @unroll on attributeMatchers
ModuleMatchers.scala	@data class → case class, @unroll on includeByDefault (replaces @since)
VersionConstraint.scala	@data class → case class for Lazy/Eager; add explicit withLatest to Eager (previously generated by @data)

build.mill changes

• Added scala3 = "3.7.4" to DepVersions
• sources task selects src-2/ or src-3/ based on Scala version
• compileMvnDeps skips data-class for Scala 3
• mvnDeps includes com.lihaoyi:::unroll-annotation:0.3.0 for Scala 3 (no compiler plugin needed with Scala 3.7+)
• scalacOptions uses -release 8 (single dash) for Scala 3 vs --release 8 for Scala 2

Binary compatibility

The @unroll annotation (from com.lihaoyi:::unroll-annotation:0.3.0) is used on Scala 3 case class parameters with default values to generate binary-compatible constructor/apply/copy overloads — replacing the role that @since played with data-class in the Scala 2 version.

[alexarchambault]

Actually, add support for 3.7.4 instead, and use the @unroll annotation

[Copilot]

Actually, add support for 3.7.4 instead, and use the @unroll annotation

Done in the latest commit:

• Switched from Scala 3.3.7 to 3.7.4
• Added com.lihaoyi:::unroll-annotation:0.3.0 as a Scala 3 dependency (no compiler plugin needed with 3.7.4)
• Applied @unroll to ModuleMatcher.attributeMatchers and ModuleMatchers.includeByDefault (which was @since in the Scala 2 version) to generate binary-compatible overloads