Use swift-embedded-foundation instead of Foundation shims

[colemancda]

Summary

Replaces the hand-rolled Foundation shims (Data, UUID, Date, Decimal, URL, ~700 lines) with the swift-embedded-foundation package for Foundation-free (Embedded Swift) builds.

• Package.swift adds the FoundationEmbedded dependency behind a SWIFT_EMBEDDED=1 environment gate, since its deployment-target minimums are higher than CoreModel's and regular builds use Foundation/FoundationEssentials anyway
• The five shim files are deleted and replaced by a single FoundationShims.swift that re-exports the FoundationEmbedded types via public typealias under #if !canImport(FoundationEssentials) && !canImport(Foundation)
• ObjectID.swift and Predicate/Expression.swift gain #elseif canImport(FoundationEmbedded) import branches, required because Embedded Swift needs a per-file import for cross-module member access
• The Embedded WebAssembly CI job now sets SWIFT_EMBEDDED=1

Testing

• swift build and swift test on macOS: 22/22 tests pass
• Embedded WebAssembly build (swift-6.3.3-RELEASE_wasm-embedded SDK) with SWIFT_EMBEDDED=1 SWIFTPM_ENABLE_MACROS=0, debug and release: builds clean

[github-code-quality]

Code Coverage Overview

Languages: Swift

Swift / code-coverage/llvm-cov

The overall coverage remains at 96%, unchanged from the master branch.

---

_{Code Coverage is in Public Preview. Learn more and provide us with your feedback.}