v0.4.0

Breaking Changes

• Expression type restructured. The public Expression type is now a
  struct with two fields — kind: ExprKind and annotations: AnnotationSet
  — rather than a flat enum. All match arms that previously destructured
  Expression::Variant { ... } must be updated to ExprKind::Variant { ... }
  and applied to expr.kind. Convenience constructors on Expression
  (Expression::integer, Expression::variable, etc.) remain available.

• Serde format changed to adjacently-tagged. The JSON wire format now uses
  { "kind": "VariantName", "value": <payload> } throughout. The previous
  externally-tagged format ({ "VariantName": <payload> }) and bare-string
  unit variants (e.g. "Nabla") are no longer emitted or accepted. Any stored
  JSON or hardcoded deserializers must be updated. See docs/WIRE-FORMAT.md
  and the migration guide below.

Added

• AnnotationSet substrate. Each Expression node now carries an
  AnnotationSet — an ordered string-to-string metadata map. The field is
  omitted from JSON when empty and defaults to empty on deserialization, so
  existing documents round-trip without modification. The substrate is passed
  through unchanged by thales v0.9.0; semantic consumption is planned for
  thales v0.10.0.

• Golden fixtures and variant stability tests. tests/fixtures/serde/
  contains one canonical JSON file per ExprKind variant (53 total) plus a
  variant_manifest.txt. CI verifies that round-tripping each fixture produces
  bit-identical output, locking the wire format against accidental drift.

• docs/WIRE-FORMAT.md. Normative wire-format reference covering all 53
  variants, the AnnotationSet field, all nested enum kind values, and the
  stability guarantee.

Migration Guide

Rust match arms. Replace Expression::Add { left, right } with:

// before
match expr {
    Expression::Binary { op, left, right } => { ... }
    Expression::Nabla => { ... }
}

// after
match expr.kind {
    ExprKind::Binary { op, left, right } => { ... }
    ExprKind::Nabla => { ... }
}

JSON format. Every node that previously serialized as
{ "VariantName": payload } now serializes as
{ "kind": "VariantName", "value": payload }. Unit variants that previously
serialized as the bare string "Nabla" now serialize as { "kind": "Nabla" }.

// before (externally tagged)
{ "Binary": { "op": "Add", "left": { "Integer": 1 }, "right": { "Integer": 2 } } }

// after (adjacently tagged)
{
  "kind": "Binary",
  "value": {
    "op":    { "kind": "Add" },
    "left":  { "kind": "Integer", "value": 1 },
    "right": { "kind": "Integer", "value": 2 }
  }
}

Swift consumers that decoded the JSON via hand-written Decodable structs must
update their CodingKeys and decoding logic to match the new shape. See
docs/WIRE-FORMAT.md for the new adjacently-tagged layout.