feat(openapi)!: migrate to oas 0.3 and OpenAPI 3.2

[Kilerd]

Gotcha still emitted OpenAPI 3.0 schemas after the release of oas 0.3. This migrates the dependency and generated documents together to OpenAPI 3.2.0. For example, Option<Record> now produces anyOf: [$ref(Record), {type: null}], so null is accepted without weakening the shared Record component.

• Centralize nullability in gotcha_core::nullable_schema; emit numeric exclusive bounds, schema examples, JSON null for Json<()>, and an unconstrained schema for raw binary bodies.
• Adapt core, derive output, parameters, responses, and components to SchemaType / SchemaValue. Preserve boolean schemas and reference siblings, finalize references inside itemSchema / contentSchema, and leave example/default payloads untouched.
• Keep Schematic in core and preserve standalone library, facade, and renamed-dependency support. Document source compatibility changes and the supported subset in MIGRATION.md.
• Default documentation endpoints to JSON and Scalar. The bundled Redoc rejects 3.2; retain its explicit configuration field, disabled by default.

Validation

All 11 GitHub CI checks passed for f91cfc2.

• Gotcha and core tests with all features and no default features; macro tests in both configurations.
• Standalone downstream fixtures, including the check that core-only consumers do not pull in web dependencies.
• Workspace formatting, Clippy with -D warnings, and rustdoc with -D warnings.
• Focused regressions for recursive nullable references, enums, reference siblings, boolean/stream-item response unions, and preserving data that resembles references.
• Exported a representative document and validated it with Python jsonschema against the official OpenAPI 3.2 schema-base. Verified acceptance/rejection of nulls, recursive values, enum values, and exclusive numeric boundaries.
• Browser verification: Scalar loads the generated 3.2 document and displays ordinary responses and examples. On 2026-09-15, the npm latest release and existing unversioned CDN URL both resolved to @scalar/api-reference 1.68.0. An explicit 1.68.0 browser probe received and displayed two real SSE events in the request client, but the documentation still showed an itemSchema-only SSE response as “No Body”. Upstream scalar/scalar#10178 adds streaming item-schema examples and remains unmerged/unreleased; there is no stable viewer upgrade for this limitation yet. Redoc fails with “Unsupported OpenAPI Version: 3.2.0”.

This establishes the schema foundation for #86. SSE annotations and WebSocket/AsyncAPI contracts remain separate; this does not claim complete OpenAPI 3.2 model or viewer support.

Closes #119.