feat: Add Embedded Swift support for SQLiteNIO

[scottmarchant]

Stacked on #4 (feat/wasi-nio-free) and retargets to base/vapor-main once that merges, so the diff here shows only the Embedded work. Research grade and lower priority than #4, which does not depend on this; review that one first.

Makes SQLiteNIO compile for wasm32-unknown-wasip1-embedded.

Changes:

• SQLiteData's Encodable conformance moves to a conditional extension and the deprecated SQLiteDataType.serialize(_:) is elided: Embedded Swift has no Encoder and no any Encodable existential.
• The async withConnection(_:) protocol requirement 1.13.0 added is elided under Embedded Swift, because a generic method requirement cannot be placed in a witness table and keeping it would make any SQLiteDatabase unusable there. The concrete SQLiteConnection.withConnection(_:) remains available.
• SQLiteError: LocalizedError, the Data bridges, and the Date bridge gate on canImport(FoundationEssentials) || canImport(Foundation), since Embedded Swift has neither flavor. The errorDescription witness stays in the type body, so only the conformance is conditional.
• The hook API gates whole-file on that same Foundation condition, and observerBuckets plus close()'s clearAllHooks() call gate with it. This one is deliberately not narrowed: the hook API's public surface is Foundation-shaped, with Date on SQLiteCommitEvent/SQLiteRollbackEvent and UUID on SQLiteHookToken.id, so keeping it would mean either shipping stand-ins for both types or giving Embedded a different public API than every other target.
• SQLiteRow.description and SQLiteCustomFunction's error reporting rely on reflection (Array.description, any Error interpolation), so reflection-free equivalents replace them under Embedded Swift.

Where the full toolchain is present, nothing changes: every gate here is false outside Embedded Swift. swift package diagnose-api-breaking-changes against #4's tip reports no breaking changes in SQLiteNIO, and the 59-test suite passes unchanged.

What Embedded loses: the Encodable conformance on SQLiteData, the deprecated SQLiteDataType.serialize(_:), the protocol-level async withConnection(_:) (the concrete one stays), SQLiteError: LocalizedError, the Data and Date bridges, and the observation hook API. Nothing here affects regular WASI, which has FoundationEssentials and Codable and needs none of it. That is why this is a separate PR.

Prerequisite outside this repo: an Embedded-clean swift-log. Upstream swift-log does not compile under Embedded Swift (Logger.Level: Codable, Dictionary.description, and Array.description are all @_unavailableInEmbedded), so the build dies inside Logging before reaching a line of sqlite-nio code. This is verified against a locally patched clone; that patch is not committed anywhere on this branch and the manifest still points at upstream apple/swift-log, so this cannot land until swift-log gains Embedded support. swift-collections needed nothing.

Verification: the Embedded build is green with swiftly run +main-snapshot-2026-06-12 swift build --swift-sdk DEVELOPMENT-SNAPSHOT-2026-06-12-a-wasm32-unknown-wasip1-embedded and the patched swift-log, exercised end to end by a downstream consumer whose Embedded test suite passes against this branch. Native and regular WASI are verified unchanged, not merely unchanged by construction: 59 tests green and no API breakage against #4.

Notes for review: every gate is a capability gate (hasFeature(Embedded) or canImport), never a platform check; there is no os(WASI) anywhere in this stack. No SwiftPM traits, no versioned manifest, no tools-version bump, no new dependencies.