Deterministic input-map rendering for KNN embedding.

Used at both compile (embedding trainset examples) and inference (embedding the live query) so the two representations land in the same vector space.

Rules

• embed_fields: :all — every key in inputs, sorted alphabetically.
• embed_fields: [:a, :b] — only those keys, in the declared order.

Each field renders as "<name>: <value>\n". Values render via:

• to_string/1 for strings, atoms (including booleans), numbers.
• Jason.encode!/1 for lists and maps.
• Raise Dsxir.Errors.Invalid.Configuration{reason: :unembeddable_input} for opaque structs that aren't enumerable.

Missing fields render as "<name>: nil\n". The renderer never raises on missing keys — that is by design.