RFE: Backend JSON codec extensibility and migration support #5995

The backend already parses JSON trees as maps/lists and generates DTO codecs with JsonProperty/JsonIgnore, enum, date, and byte-array support. Generated bodies require a no-argument constructor, generic type-variable fields and non-byte arrays are rejected, and unknown members are always ignored. More complex API contracts need explicit extension points instead of a runtime Jackson dependency.

Requested capability

• Support registered build-time custom codecs and construction strategies for immutable DTOs.
• Extend generated handling of typed generic DTOs and arrays, with explicit diagnostics for remaining unsupported shapes.
• Expose configurable unknown-field handling, aliases, null/absent semantics, and date/numeric formatting policies.
• Provide a documented migration guide for common ObjectMapper/JsonNode use cases using the existing tree API and generated codecs.

Acceptance criteria

• Contract fixtures cover renamed/ignored fields, nested generics, arrays, immutable values, unknown members, and absent versus explicit null.
• Custom codecs are retained in native output without reflection or runtime scanning.
• Malformed values retain precise field-path diagnostics and configurable resource limits.
• Existing default JSON behavior remains backward compatible.

Current implementation

• vm/backend/src/com/codename1/backend/Json.java
• maven/build-engine/src/main/java/com/codename1/maven/processors/BackendJsonCodecs.java
• docs/developer-guide/Backend-Web.asciidoc

Validate behavior on the JVM and locally on the packaged ParparVM backend where applicable. This request does not call for native-image or GraalVM CI jobs.