An earlier entry of an `extends` array takes precedence over a later one for `cmkOptions`

Summary

When extends in tsconfig.json is an array, cmkOptions of an earlier entry take precedence over those of a later entry. TypeScript resolves the same conflict in the opposite way: the later entry wins.

Details

tsconfig.base1.json:

{
  "compilerOptions": { "module": "esnext" },
  "cmkOptions": { "dtsOutDir": "generated1" }
}

tsconfig.base2.json:

{
  "compilerOptions": { "module": "es2015" },
  "cmkOptions": { "dtsOutDir": "generated2" }
}

tsconfig.json:

{
  "extends": ["./tsconfig.base1.json", "./tsconfig.base2.json"]
}

Result of readConfigFile:

readConfigFile merges the options of the extended files in the order TypeScript lists them, and an option that is already set is never overwritten. This gives the right precedence for a chain (tsconfig.json → base2 → base1), but the wrong one for an array.

Expected behavior

dtsOutDir is <rootDir>/generated2. For every option of cmkOptions, a later entry of the extends array takes precedence over an earlier one, as TypeScript does for compilerOptions.

🤖 Generated with Claude Code

Start at readConfigFile, using the three tsconfig.json examples in the issue to reproduce the precedence difference between an extends array and a chain. Inspect how extended options are merged, then verify that the later array entry supplies dtsOutDir while preserving TypeScript's compilerOptions behavior. Done means the result resolves dtsOutDir to /generated2.