fix: valid specs lose data, duplicate keys, or fail to compile

[giraffesyo]

Follow-up review of #36, #40, and #41. Seven defects in shipped behavior, four of them data-losing or compile-breaking.

Regression introduced by #41

A discriminator with no explicit mapping now silently loses every payload. The analyzer only fills Discriminator.Mapping from an explicit mapping key, so an implicit-mapping union generated a switch with zero cases and only a default. Before #41 that default returned an error — loud and wrong. After #41 it returns nil with Value == nil, so {"kind":"cat","lives":9} decodes "successfully" into nothing.

The mapping is now derived from the variant schema names, which is what OpenAPI says the implicit values are. This is the one finding that is strictly worse than before my change, and it's the reason this PR exists.

Data loss and corruption in the catch-all (from #40)

additionalProperties: false was treated as true. resolveAdditionalPropertiesType never checked ap.B, and convertObject only tested != nil. A schema that explicitly forbids unknown properties got a catch-all field plus marshalers that collected and re-emitted them. New allowsAdditionalProperties is used at all three sites; type SealedEmpty struct{} still compiles.

Declared keys were deleted by exact match, but encoding/json matches tags case-insensitively. Schema {a: string, additionalProperties: true} receiving {"A":"hello"}: the stdlib fills A into the declared field via its case-insensitive fallback, delete(obj, "a") doesn't remove "A", so it also lands in the catch-all — and re-marshals as {"A":"hello","a":"hello"}. One logical property, duplicated on the wire. Both marshalers now share a generated deleteDeclaredProperties helper using strings.EqualFold.

*t = T(s) discarded fields the payload omitted. json.Unmarshal into a non-zero value merges for every generated struct except the ones that got custom marshalers, which silently replaced the whole value and kept stale catch-all entries. Now decodes through (*shadow)(t) after clearing the map — stdlib semantics, and one struct copy fewer.

Compile break (from #40)

A schema with a property named additionalProperties emitted two fields of that name: types.go:100:2: AdditionalProperties redeclared, plus five more errors in the marshalers. catchAllFieldName now picks AdditionalProperties2 when the name is taken.

Correctness and quality

• null and missing-discriminator payloads fell into the unknown-variant branch, so a caller couldn't distinguish "server added a variant" from "malformed body". null is now a no-op and a missing discriminator is an error (the case "" arm is suppressed when a mapping legitimately maps "").
• Every response shared one name hint, so two inline union bodies on one operation collided into DoThingResponse2 — and inserting a 201 later would rename the 400 type. Non-success bodies now carry their status code; DoThingResponse2 became DoThingResponse400.
• MarshalJSON made three JSON passes (marshal → unmarshal to map → marshal) and reordered declared properties alphabetically as a side effect. Now splices the two encoded objects: one pass fewer, struct field order preserved.
• The new e2e test re-implemented scaffolding that generateFromSpec/runGeneratedWireTest already provide; rewritten on top of them (~60 duplicated lines gone). Four WHAT comments on unexported template helpers removed per CLAUDE.md.

Deliberately not fixed

• Typed additionalProperties with a mismatched value fails the whole decode. {owner: string, additionalProperties: {type: string}} receiving {"count":3} errors out and loses the response — the opposite of the resilience fix: an unknown discriminator value no longer fails the whole decode #41 argues for. Whether to error or skip a spec-violating value is a product call; the case-insensitivity fix removes the common false trigger. Say the word and I'll make it skip.
• allOf still ignores additionalProperties entirely. convertAllOf never emitted a catch-all, so composed schemas drop unknown properties — fix: additionalProperties are dropped on decode and sent as a "-" key #40's bug, unfixed for that shape. The fix isn't just adding the field: type shadow T would promote an embedded parent's marshalers and break encoding, and declaredJSONNames skips embedded fields so promoted properties would leak into the catch-all. Needs the marshaler generalized to embedded types; larger than this PR.
• Discriminated unions are no longer comparable. Adding raw json.RawMessage in fix: an unknown discriminator value no longer fails the whole decode #41 means shape == Shape{} stops compiling downstream. Inherent to preserving the payload — flagging it as a breaking-change note for the release, not a code fix, since fix: an unknown discriminator value no longer fails the whole decode #41 shipped as fix:.

Tests

Runtime e2e coverage added for the case-insensitive leak, catch-all reset, and additionalProperties: false; the inline-union e2e extended with missing-discriminator and null cases. gofmt, go vet, and go test ./... pass.

Provenance

Produced by /code-review xhigh --fix over 4c237be..ff8808c. The fixes were extracted onto a clean branch off canary because a concurrent session had unrelated in-flight work (non-JSON request bodies, enum naming, nullable-union collapsing) interleaved in the same files; none of it is included here.