Support .archify diagram files (0.2.0)

.archify files are Archify JSON diagram sources. They open as JSON,
preview with the same commands, report validation problems, and get
completion from a schema that dispatches on diagram_type. Adds a
hello.archify example and tests for detection, the dispatching schema
and the VS Code behaviour.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/schema.test.ts

@@ -0,0 +1,43 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import { test } from 'node:test';
+import Ajv2020 from 'ajv/dist/2020';
+
+const root = path.resolve(__dirname, '..', '..');
+const schemas = path.join(root, 'schemas');
+const fixtures = path.join(root, 'test', 'fixtures');
+
+function validator() {
+  const ajv = new Ajv2020({ strict: false, allErrors: true });
+  // Register every schema under its file name, exactly how the relative
+  // $refs in the bundled copies resolve inside VS Code.
+  for (const file of fs.readdirSync(schemas)) {
+    ajv.addSchema(JSON.parse(fs.readFileSync(path.join(schemas, file), 'utf8')), file);
+  }
+  return ajv.getSchema('archify.schema.json')!;
+}
+
+test('bundled schemas carry no absolute $id', () => {
+  for (const file of fs.readdirSync(schemas)) {
+    assert.equal(JSON.parse(fs.readFileSync(path.join(schemas, file), 'utf8')).$id, undefined, file);
+  }
+});
+
+test('the .archify schema accepts every fixture', () => {
+  const validate = validator();
+  for (const file of fs.readdirSync(fixtures)) {
+    const ok = validate(JSON.parse(fs.readFileSync(path.join(fixtures, file), 'utf8')));
+    assert.ok(ok, `${file}: ${JSON.stringify(validate.errors?.slice(0, 3))}`);
+  }
+});
+
+test('the .archify schema applies the schema of the declared type', () => {
+  const validate = validator();
+  const sequence = JSON.parse(fs.readFileSync(path.join(fixtures, 'cache-miss.archify'), 'utf8'));
+  // A sequence diagram is not a valid architecture diagram.
+  assert.equal(validate({ ...sequence, diagram_type: 'architecture' }), false);
+  assert.equal(validate({ ...sequence, diagram_type: 'poster' }), false);
+  const { diagram_type: _omitted, ...untyped } = sequence;
+  assert.equal(validate(untyped), false);
+});