Handle item replacement directories in FoundationEssentials

[Hokila]

Motivation

Resolves #991.

FoundationEssentials currently falls through to normal search path lookup for .itemReplacementDirectory. That lookup returns no URL, so FileManager.url(for:in:appropriateFor:create:) throws on Linux when callers ask for a replacement directory.

Modifications

Add a FoundationEssentials path for .itemReplacementDirectory when the domain is .userDomainMask and appropriateFor is supplied. It creates a TemporaryItems container under the process temporary directory when that directory is on the same volume as the reference URL. Otherwise it falls back to a writable directory based on the reference URL, matching the older corelibs behavior.

The replacement directory itself is always created, including for create: false, so callers get a unique directory and avoid races.

Add a regression test that covers create: false and create: true, verifies the returned directories exist, and checks that repeated calls return distinct directories.

Validation

• git diff --check
• swiftc -parse Sources/FoundationEssentials/FileManager/FileManager+Directories.swift Tests/FoundationEssentialsTests/FileManager/FileManagerTests.swift

I could not run swift test --filter FileManager/itemReplacementDirectory locally with Xcode Swift 6.2.3. The package starts building on the release/6.4.x base, but fails in existing source that requires a newer compiler (@specialized and @_lifetime diagnostics in String+Internals.swift) before it reaches this test.

[Hokila]

Reopened as #2220 after renaming the fork branch to remove the codex prefix.