Embedded Swift support

[colemancda]

Overview

Removes the runtime Codable-based TLVEncoder/TLVDecoder and replaces it with explicit TLV coding compatible with Embedded Swift, following the same design as CoreModel.

Changes

• TLVEncodable / TLVDecodable no longer refine Encodable / Decodable; conformances implement var tlvData: Data and init?(tlvData:) directly.
• New TLVContainer type for keyed encoding and decoding of structured types via TLVCodingKey.
• Primitive conformances for integers, Bool, Float, Double, String, Data, and arrays, plus RawRepresentable default implementations. Wire format is unchanged: little-endian, Int/UInt as 32-bit, arrays as nested containers keyed by index.
• New @TLVCodable macro (TLVCodingMacros target) generates CodingKeys, init?(tlvData:), and tlvData; a hand-written CodingKeys enum with explicit type codes is respected. Conformances can also be written by hand (required under Embedded Swift).
• Minimal Data shim for platforms without Foundation; UUID and Date conformances gated on Foundation availability.
• Package manifest updated to Swift tools 6.0, with swift-syntax gated behind the SWIFTPM_ENABLE_MACROS environment variable.
• Tests rewritten against the new API, preserving the previous byte-exact wire format vectors, plus macro expansion tests.
• CI updated with an Embedded Swift job:

SWIFTPM_ENABLE_MACROS=0 swift build --target TLVCoding \
    --triple armv7em-none-none-eabi \
    -Xswiftc -enable-experimental-feature -Xswiftc Embedded \
    -Xswiftc -wmo

Breaking changes

• TLVEncoder, TLVDecoder, formatting options (TLVDateFormatting, TLVUUIDFormatting, TLVNumericFormatting, TLVOutputFormatting), and dictionary support are removed.
• Encoding is fixed to the previous defaults: little-endian, UUID as 16 bytes, Date as seconds since 1970.