Prepare for tagging 1.3.0

[lorentey]

[This is an updated version of #522, with the source branch within the same repository.]

• Review RigidArray, UniqueArray APIs; split implementation into multiple files
• Set the availability of these new types to match the Swift 5.0-aligned OS releases. (We can loosen this later, but we want to use spans in the implementation of most nontrivial operations, and spans are only available from >=5.0 runtimes.)
• Use consistent indentation & add file headers in TrailingElementsModule
• Disable UnstableContainersPreview package trait while we're debugging a runtime crash
• Rename struct Borrow to struct Ref, and struct Inout to struct Mut.

This also includes #520, in order to (potentially) speed up validation work.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

CI issues:

• The Xcode 26.0 job is unable to run our tests as SwiftPM is unable to load libswiftCompatibilitySpan.dylib.
• Something somewhere requires a successful run with Xcode 16.2, but that appears to be not available for CI testing.