defineConfig and mergeConfig vanished from stryker-js/config in 11.0.0

[systemfsoftware-maker]

Problem Statement

Upgrading @systemfsoftware/stryker-js from 10.x to 11.0.0 breaks every stryker.config.ts that follows the documented 10.x pattern: import { defineConfig } from '@systemfsoftware/stryker-js/config' now fails at load time, so the run dies before it reads a single option. The 11.0.0 CHANGELOG lists only a patch fix, so nothing tells the user why. The replacement, StrykerConfig.define(…) / StrykerConfig.merge(…), is more Effect-like, but it breaks the ergonomics every JS tool config shares (defineConfig / mergeConfig from Vite, Vitest, tsdown, oxlint), and it changes what the StrykerConfig type means, so the common const config: StrykerConfig = defineConfig({…}) annotation breaks too. The 10.x ./config surface should come back.

Goal

The next published @systemfsoftware/stryker-js release exports defineConfig and mergeConfig from ./config with their 10.1.1 signatures and exports StrykerConfig as the partial-options type, so a stryker.config.ts written against 10.1.1's ./config surface typechecks and loads without edits.

Evidence

A consumer config unchanged from 10.x, loaded by 11.0.0 (dry run through @systemfsoftware/stryker-js/promises):

_tag: 'ConfigFileUnreadableError',
file: 'stryker.config.ts',
  message: 'Failed to import module "file:///…/packages/effect-readiness/stryker.config.ts"',
  cause: …/effect-readiness/stryker.config.ts:2
  import { defineConfig,                    } from '@systemfsoftware/stryker-js/config'
           ^^^^^^^^^^^^
  SyntaxError: The requested module '@systemfsoftware/stryker-js/config' does not provide an export named 'defineConfig'

The published ./config declarations, 10.1.1 against 11.0.0:

// 10.1.1 dist/config.d.mts:10-23
type StrykerConfig = PartialStrykerOptions;
export declare function defineConfig(config: StrykerConfig): StrykerConfig;
export declare const mergeConfig: (defaults: StrykerConfig, overrides: StrykerConfig) => StrykerConfig;
export type { ConfigEnv, PartialStrykerOptions, StrykerConfig, StrykerConfigExport, StrykerConfigFn, StrykerOptions };

// 11.0.0 dist/config.d.mts:7 and dist/stryker-config.schema-lr7GMjpY.d.mts:17-21
export { type ConfigEnv, type Immutable, type ImmutablePrimitive, type Primitive, StrykerConfig, type StrykerConfigExport, type StrykerConfigFn };
declare const StrykerConfig_base: S.Class<StrykerConfig, S.Struct<{ readonly entries: S.$Record<S.String, S.Unknown> }>, {}>;
declare class StrykerConfig extends StrykerConfig_base {
  static define(config: mod_d_exports$2.PartialStrykerOptions): mod_d_exports$2.PartialStrykerOptions;

In 11.0.0, StrykerConfig as a type is the class instance { entries: Record<string, unknown> }, so a PartialStrykerOptions value no longer fits a StrykerConfig annotation.

Orientation

• The change landed in refactor(repo): move every package to the compound-pack bar #94 (refactor(repo): move every package to the compound-pack bar), which moved the helpers onto the class in packages/stryker-js/src/config/stryker-config.schema.ts at ff66457. The 10.x helpers lived in src/config/define-config.ts and src/config/merge-config.ts; the entry point is src/config/mod.ts.
• The changeset that describes the removal, .changeset/stryker-js-config-and-exports.md (major), is still unconsumed on main at ff66457. It also announces other removals (createDefaultOptions, SUPPORTED_CONFIG_FILE_NAMES, calculateMetrics, and others) that are out of scope here.
• packages/stryker-js/README.md documents StrykerConfig.define / StrykerConfig.merge as the config API, as does the README of every package whose example uses them.
• The package already runs typecheck (tsc -b), test (vitest run) and api:check (api-extractor run).

Non-Counting Outcomes

• A CHANGELOG or migration note that documents the removal. The ask is to restore the ergonomic API; documenting the break keeps it broken.
• Restoring defineConfig and mergeConfig as runtime exports while StrykerConfig remains the class, so the runtime import resolves but const config: StrykerConfig = defineConfig({…}) still fails to typecheck.
• Restoring the exports in source while the pending changeset still ships, so the next CHANGELOG announces the removal of defineConfig / mergeConfig that the same release reverts.
• A test that imports defineConfig from src/config/mod.ts only, while the built ./config entry point (dist/config.mjs / dist/config.d.mts) still lacks the export.
• Restoring defineConfig with a narrower signature than 10.1.1: it must accept a plain object, a promise, and a (env: ConfigEnv) => … factory, as the four 10.1.1 overloads do.

Acceptance Criteria

• pnpm --filter @systemfsoftware/stryker-js typecheck and test exit 0, including a check that fails on ff66457: a config module importing { defineConfig, mergeConfig, type StrykerConfig } from @systemfsoftware/stryker-js/config with const config: StrykerConfig = defineConfig({…}) and mergeConfig(base, overrides) typechecks and loads (gatekeeper).
• The built dist/config.mjs exports defineConfig and mergeConfig as named exports, and dist/config.d.mts declares the four defineConfig overloads, mergeConfig(defaults, overrides), and StrykerConfig as the partial-options type (inspect pnpm --filter @systemfsoftware/stryker-js build output).
• No unconsumed changeset on main announces the removal of defineConfig or mergeConfig; the release that restores them carries a CHANGELOG entry saying so.
• packages/stryker-js/README.md and every package README that shows a Stryker config example use defineConfig / mergeConfig from ./config.