Fix Embedded Swift support for Entity decoding

[colemancda]

Summary

Adds proper Embedded Swift support for the generic Entity-based decode path. The library target already compiled standalone under Embedded and the swift-wasm CI job passed — but neither ever instantiated the generic Entity.init(from:)/ModelData.decode path for a concrete conforming type (those types live in the test target, which the embedded job doesn't build). So the bug was latent: the moment a downstream package declares a concrete Entity and calls init(from:) under Embedded, it fails with:

error: cannot use a value of protocol type 'any Error' in embedded Swift [#EmbeddedRestrictions]

Root cause

Two places throw a concrete CoreModelError from a function declared as untyped throws (sugar for throws(any Error)). Under Embedded Swift, boxing a concrete error into any Error is disallowed, so the compiler rejects it once the path is specialized for a real type:

1. Entity.init(from:) — the protocol requirement itself is untyped throws, forcing a boxing thunk in every throwing witness's table entry.
2. ModelData.decode/decodeRelationship — plain throws, but under Embedded they call factories returning concrete CoreModelError.

Error.swift already returned the concrete type conditionally under Embedded — it just never propagated that into the throws clauses of its callers.

Fix

Introduce one typealias — ModelDataDecodingError — resolving to CoreModelError under Embedded and any Swift.Error otherwise, and use it as the throws(...) type on the requirement and the four generic decode helpers. On non-Embedded platforms throws(ModelDataDecodingError) is byte-for-byte identical to the old plain throws: zero behavior change, no API break for existing conformers on Darwin/Linux/Windows.

One spot needed more than a signature swap: the objectIDs.map { throw … } in decodeRelationship(_:forKey:) for to-many goes through the rethrows overload, which erases the thrown type back to any Error. It's rewritten as an explicit loop so the typed error propagates.

encode() is left untyped — every witness is non-throwing today, so there's nothing to box.

Regression coverage

Adds an always-built, Embedded-gated concrete Entity fixture (EmbeddedFixture.swift) that forces the compiler to specialize the generic decode path — the exact instantiation CI was missing. Without it, this regression can silently reappear.

Out of scope

The other Embedded caveat — ModelStorage's generic fetch/insert/delete/count and ViewContext — is a separate upstream compiler bug (swiftlang/swift#78811), unrelated to error-boxing, and is not touched here.

Verification

• swift build --target CoreModel ✅
• swift build --target CoreModel --swift-sdk swift-6.3.3-RELEASE_wasm-embedded ✅ (fails without the loop fix)
• swift build (full package) ✅
• swift test ✅ 22 tests, 7 suites

[github-code-quality]

Code Coverage Overview

Languages: Swift

Swift / code-coverage/llvm-cov

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

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