Embedded Swift support for SQLKit

[scottmarchant]

Makes SQLKit compile under Embedded Swift. This builds on #3 (feat/wasi-nio-free) and retargets to base/vapor-main once that merges, so this diff shows only the Embedded work. It is research grade and lower priority than #3, which does not depend on it; if this direction isn't wanted, dropping it costs the WASI support nothing.

The part that needs real discussion is the bound-parameter type. Embedded Swift has no Codable, and SQLKit's bound-parameter constraint is spelled Encodable across roughly eighty public declarations, so the constraint becomes a typealias that resolves differently there:

#if hasFeature(Embedded)
public protocol SQLBindValue { var sqlDataValue: SQLDataValue { get } }
public typealias SQLBindable = SQLBindValue
#else
public typealias SQLBindable = Encodable
#endif

Off Embedded, SQLBindable is Encodable, so every some/any Encodable & Sendable that now reads some/any SQLBindable & Sendable is the same type as before. swift package diagnose-api-breaking-changes reports no breaking changes in SQLKit (the same check the required unit-tests / api-breakage job runs), and the 169-test suite is untouched and green. What remains is that it reads like a large mechanical diff across thirty files, and I know that's a lot to ask of a reviewer; the alternative, duplicating every one of those declarations under a gate, seemed worse. Happy to be told no.

The rest of the Embedded work:

• The Codable engine (SQLQueryEncoder, SQLRowDecoder, SomeCodingKey, the SQLCodingUtilities helpers, and the model-shaped builder overloads) gates out under Embedded Swift.
• The generic withSession(_:) moves from protocol requirement to extension default there, since a generic requirement can't be placed in a witness table under Embedded Swift without making any SQLDatabase unusable.
• Reflection and existential-cast paths (the SQLDatabaseReportedVersion comparisons, description formatting) get Embedded-safe equivalents, implicit array upcasts are boxed explicitly, and one key-path literal becomes a closure.
• StringHandling's trimmingPrefix(_:) and regex-based helpers fall through to the existing hand-rolled implementations, since the Embedded stdlib ships neither.
• SQLKitBenchmark's contents gate on !hasFeature(Embedded) (it needs XCTest and Codable), not on any platform; regular WASI still compiles the benchmark target, verified.

Same prerequisite as the sqlite-nio Embedded PR: an Embedded-clean swift-log. Upstream 1.14.0 does not compile under Embedded Swift and the build dies inside Logging first. Verified against a locally patched clone; the patch is not committed here and the manifest still points at upstream apple/swift-log. swift-collections needed nothing.

Verified locally: native build and 169 tests green, no API breakage against the base, regular WASI green, and the Embedded build green (DEVELOPMENT-SNAPSHOT-2026-06-12-a-wasm32-unknown-wasip1-embedded with the patched swift-log).