feat(core): stabilize programmatic JS API with createRstest + /api surface

Move all programmatic entrypoints to `@rstest/core/api`; the main
`@rstest/core` entry now exposes only in-test runtime globals, config
helpers (defineConfig/loadConfig/merge*), and types.

- Replace the standalone `runRstest` with an async `createRstest()`
  factory (rsbuild-`createRsbuild`-style) returning an `RstestInstance`
  with `run`/`listTests`/`mergeReports`/`close`. Construction carries
  static host wiring + config: `configFile` (a path to load) and `config`
  (an inline override). `config` accepts either an object (deep-merged
  over the disk config) or a `(loadedConfig) => RstestConfig` callback
  that receives the resolved disk config and returns the final config —
  letting callers transform it directly instead of merging.
  Per-invocation selection/control lives in `RunOptions`
  (filters/related/changed/shard/bail/...).
- Add a jest-`runCLI`-aligned public `runCli(argv, { cwd })` that takes a
  parsed-object argv (positionals in `argv._`), stays host-safe (never
  calls `process.exit`), and returns a structured `TestRunResult`.
- Keep the raw-argv CLI router (`startCli`) off the public surface: build
  it as its own internal `dist/cli.js` chunk (not a `package.json`
  export) and have the `rstest` bin import that artifact directly,
  mirroring how vitest builds `src/node/cli.ts` -> `dist/cli.js`.
- Rename the internal sync factory `createRstest` -> `createRstestContext`
  and the internal type `RstestInstance` -> `RstestRunner`; share public
  result types + assembly helpers via `src/api/result.ts`, and dedupe the
  filter-resolve/context-build tail into `buildResolvedRunner`.
- Migrate the vscode worker and the programmatic e2e fixtures.

BREAKING CHANGE: programmatic APIs are no longer exported from
`@rstest/core` (`createRstest`, `initCli`, `runCLI` removed from the main
entry). Import them from `@rstest/core/api`. `runRstest(opts)` is removed
-- use `const r = await createRstest(opts); await r.run()`.

Author: fi3ework

e2e/programmatic/fixtures/config-fn/a.test.ts

@@ -0,0 +1,5 @@
+import { expect, it } from '@rstest/core';
+
+it('a passes', () => {
+  expect(1).toBe(1);
+});

e2e/programmatic/fixtures/config-fn/b.test.ts

@@ -0,0 +1,5 @@
+import { expect, it } from '@rstest/core';
+
+it('b passes', () => {
+  expect(2).toBe(2);
+});

e2e/programmatic/fixtures/config-fn/rstest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from '@rstest/core';
+
+// Disk config includes BOTH test files; the programmatic `config` callback
+// receives this resolved object and narrows `include` to a single file.
+export default defineConfig({
+  include: ['a.test.ts', 'b.test.ts'],
+  reporters: [],
+});

e2e/programmatic/fixtures/related/math.test.ts

@@ -0,0 +1,8 @@
+import { describe, expect, it } from '@rstest/core';
+import { add } from './src/math';
+
+describe('math', () => {
+  it('adds', () => {
+    expect(add(1, 2)).toBe(3);
+  });
+});

e2e/programmatic/fixtures/related/rstest.config.ts

@@ -0,0 +1,6 @@
+import { defineConfig } from '@rstest/core';
+
+export default defineConfig({
+  include: ['**/*.test.ts'],
+  reporters: [],
+});

e2e/programmatic/fixtures/related/src/math.ts

@@ -0,0 +1 @@
+export const add = (a: number, b: number): number => a + b;

e2e/programmatic/fixtures/related/unrelated.test.ts

@@ -0,0 +1,7 @@
+import { describe, expect, it } from '@rstest/core';
+
+describe('unrelated', () => {
+  it('does not import the math source', () => {
+    expect(true).toBe(true);
+  });
+});

e2e/programmatic/fixtures/run-config-fn.mjs

@@ -0,0 +1,34 @@
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { createRstest } from '@rstest/core/api';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const cwd = join(__dirname, 'config-fn');
+
+// Function-form `config`: the callback receives the resolved disk config and
+// returns the final config — here it narrows `include` from two files to one.
+const rstest = await createRstest({
+  cwd,
+  configFile: 'rstest.config.ts',
+  config: (loaded) => {
+    // Prove the disk config was loaded and handed in before transforming it.
+    if (!loaded.include?.includes('b.test.ts')) {
+      throw new Error(
+        `config callback did not receive disk config; got include=${JSON.stringify(loaded.include)}`,
+      );
+    }
+    return { ...loaded, include: ['a.test.ts'] };
+  },
+});
+const result = await rstest.run();
+await rstest.close();
+
+console.log(
+  `__RSTEST_API_RESULT__${JSON.stringify({
+    ok: result.ok,
+    stats: result.stats,
+    files: result.files
+      .map((f) => ({ status: f.status, testPath: f.testPath.split('/').pop() }))
+      .sort((a, b) => a.testPath.localeCompare(b.testPath)),
+  })}__END__`,
+);

e2e/programmatic/fixtures/run-failing.mjs

@@ -1,17 +1,19 @@
 import { fileURLToPath } from 'node:url';
 import { dirname, join } from 'node:path';
-import { runRstest } from '@rstest/core/api';
+import { createRstest } from '@rstest/core/api';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const cwd = join(__dirname, 'disk');
 
-const result = await runRstest({
+const rstest = await createRstest({
   cwd,
-  inlineConfig: {
+  config: {
     include: ['failing.test.ts'],
     reporters: [],
   },
 });
+const result = await rstest.run();
+await rstest.close();
 
 console.log(
   `__RSTEST_API_RESULT__${JSON.stringify({

e2e/programmatic/fixtures/run-inline.mjs

@@ -1,18 +1,20 @@
 import { fileURLToPath } from 'node:url';
 import { dirname, join } from 'node:path';
-import { runRstest } from '@rstest/core/api';
+import { createRstest } from '@rstest/core/api';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const cwd = join(__dirname, 'disk');
 
-const result = await runRstest({
+const rstest = await createRstest({
   cwd,
-  inlineConfig: {
+  config: {
     include: ['*.test.ts'],
     exclude: ['failing.test.ts'],
     reporters: [],
   },
 });
+const result = await rstest.run();
+await rstest.close();
 
 console.log(
   `__RSTEST_API_RESULT__${JSON.stringify({