Issue #168 port jtd-esm-codegen onto the incubator main

Port the jtd-esm-codegen module from the jdt-to-esm branch onto current
main (parent 2026.08.31, jdk.incubator.java.util.json API). The module
generates standalone ES2020 ESM validator modules from a flat JTD
(RFC 8927) subset for browser consumers; its test suite retains 359
tests (316 official validation.json conformance via GraalJS, 31 unit,
2 jqwik property, 10 junit-js) with 316/316 conformance preserved.

Mechanical adaptations applied:
- Module pom parent version 0.1.9 -> 2026.08.31.
- Package move jdk.sandbox.java.util.json -> jdk.incubator.java.util.json
  (JtdParser, JtdEsmConformanceTest, JtdEsmPropertyTest); the three
  unused JSON imports in JtdToEsmCodegenTest were deleted.
- API renames: members() -> asMap(), bool() -> asBoolean(),
  string() -> asString(), elements() -> asList(), element(i) -> get(i),
  Json.toDisplayString(x, 0) -> (x, "").
- Root pom: jtd-esm-codegen registered as an unconditional reactor
  module (release target 21; GraalJS is test-scoped only).
- .gitignore: removed the jtd-esm-codegen/ ignore entry.
- ci.yml exp_tests: 1679 -> 2038, measured empirically from
  surefire/failsafe XML totals after a full-reactor verify
  (matches Agent 1's estimate of 1679 + 359).
- README: module-table row + "JTD to ESM Validator Codegen
  (Experimental)" section; AGENTS: module registry entry.

The nightly release workflow (jtd-esm-codegen-release.yml) is
deliberately deferred to a follow-up.

Verify with:
  mvn -pl jtd-esm-codegen -am clean test -Djava.util.logging.ConsoleHandler.level=INFO
  (359 tests, 0 failures/errors)
  mvn clean verify -Djava.util.logging.ConsoleHandler.level=INFO
  (BUILD SUCCESS; reactor XML total 2038 tests, 0 failures/errors)

Author: simbo1905

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
                       for k in totals: totals[k]+=int(r.get(k,'0'))
                   except Exception:
                       pass
-          exp_tests=1679
+          exp_tests=2038
           exp_skipped=0
           if totals['tests']!=exp_tests or totals['skipped']!=exp_skipped:
               print(f"Unexpected test totals: {totals} != expected tests={exp_tests}, skipped={exp_skipped}")

.gitignore

@@ -41,4 +41,3 @@ pom.xml.versionsBackup
 jtd*.log
 **/jtd*.log
 .DS_Store
-jtd-esm-codegen/

AGENTS.md

@@ -152,6 +152,7 @@ See `README.md` for user-facing commands. When running locally as an agent, use
 - `json-compatibility-suite`: JSON Test Suite compatibility validation.
 - `json-java21-jtd`: JSON Type Definition (JTD) validator based on RFC 8927.
 - `json-java21-jsonpath`: JsonPath query engine over `jdk.incubator.java.util.json` values.
+- `jtd-esm-codegen`: Experimental JTD (RFC 8927) to ES2020 ESM validator code generator for JS consumers; 316/316 official validation.json conformance. See its README and `JTD_CODEGEN_SPEC.md` before working on it.
 
 Only when you are asked to work on a specific module, start by reading that module's `README.md`, then its `AGENTS.md`.
 

README.md

@@ -36,6 +36,7 @@ This repo is organized into the following modules:
 | `json-java21-jsonpath` | JsonPath query engine over `jdk.incubator.java.util.json` values (Goessner-style: filters, slices, recursive descent, unions) | 21+ |
 | `json-compatibility-suite` | JSON Test Suite conformance reporter (tests against [nst/JSONTestSuite](https://github.com/nst/JSONTestSuite)) | 21+ |
 | `json-java21-api-tracker` | Daily upstream API drift detector — fetches OpenJDK sandbox sources, compares public API signatures, reports differences | 25+ |
+| `jtd-esm-codegen` | Experimental JTD → ES2020 ESM validator code generator for JS/browser consumers | 21+ |
 
 We welcome contributions to the incubating modules.
 
@@ -413,6 +414,40 @@ An optional `jtd2jar` CLI tool and distroless Docker image are available for pre
 
 See [`jtd2jar/README.md`](jtd2jar/README.md) for build instructions, container usage, and the pre-built image on GitHub Container Registry (`ghcr.io`).
 
+## JTD to ESM Validator Codegen (Experimental)
+
+This repo also contains an **experimental** CLI tool that reads a JTD schema (RFC 8927) and generates a **vanilla ES2020 module** exporting a `validate(instance)` function. The intended use case is validating JSON event payloads in the browser (for example, across tabs using `BroadcastChannel`) without a build step. The generator passes the full official JTD conformance suite (316/316 cases from `validation.json`) and its output is executed in GraalJS during the test run.
+
+### Supported JTD subset (flat schemas only)
+
+This tool deliberately supports only:
+- `properties` (required properties)
+- `optionalProperties`
+- `type` primitives (`string`, `boolean`, `timestamp`, `int8`, `int16`, `int32`, `uint8`, `uint16`, `uint32`, `float32`, `float64`)
+- `enum`
+- `metadata.id` (used for the output filename prefix)
+
+It rejects other JTD features (`elements`, `values`, `discriminator`/`mapping`, `ref`/`definitions`) and also rejects **nested `properties`** (object schemas inside properties).
+
+When rejected, the error message is:
+
+`Unsupported JTD feature: <feature>. This experimental tool only supports flat schemas with properties, optionalProperties, type, and enum.`
+
+### Build and run
+
+```bash
+./mvnw -pl jtd-esm-codegen -am package
+java -jar ./jtd-esm-codegen/target/jtd-esm-codegen.jar schema.jtd.json
+```
+
+The output file is written to the current directory as:
+
+`<metadata.id>-<sha256_prefix_8>.js`
+
+Where `<sha256_prefix_8>` is the first 8 characters of the SHA-256 hash of the input schema file bytes.
+
+See [`jtd-esm-codegen/JTD_CODEGEN_SPEC.md`](jtd-esm-codegen/JTD_CODEGEN_SPEC.md) for the generated-code specification.
+
 ## Building
 
 Requires JDK 21 or later. Build with Maven: