please-style: choose a shared-helper location for Kotlin Multiplatform modules #49

Follow-up from #45 (PR #48)

PR #48 taught please-style check to recognize Kotlin Multiplatform modules. The test-helpers-in-dedicated-location check now skips them, because the only helper location it knows for Kotlin/Java is Gradle's src/testFixtures/, and multiplatform modules have no such source set (java-test-fixtures does not apply to KMP source sets).

Without a skip, the check would tell a multiplatform module to move a shared helper (for example a public FakeClock in src/commonTest/kotlin/…, imported by two other tests) to src/testFixtures/, a finding nobody can fix. So shared helpers in multiplatform modules are not checked at all right now.

Current behavior

• packages/cli/src/check/layouts.ts: jvmLayout returns no helper dirs for a multiplatform unit (helperDirs: unit => unit.multiplatform === true ? [] : ['src/testFixtures']).
• packages/cli/src/check/check-helpers.ts: inspectHelpers skips a candidate whose unit has no helper dirs.
• Both READMEs (packages/cli/README.md, README.ko.md) say the helper check skips multiplatform modules.

Decide

Choose where shared test helpers live in a Kotlin Multiplatform module, then enforce it. Candidates:

1. A package inside commonTest, for example src/commonTest/kotlin/**/testing/. Works with no extra Gradle setup, but a helper is visible only to the module's own tests.
2. A dedicated test-support module, for example :<module>-testing with its helpers in commonMain. Other modules can share it, but the checker would have to tie that module to its consumers.
3. A custom source set such as commonTestFixtures, wired with dependsOn. Closest to src/testFixtures, but it is not a Gradle or Kotlin convention, so each project has to configure it by hand.

The choice probably belongs in ADR-0022 (engineering-standards) next to the existing src/testFixtures/ rule for Kotlin/Java.

Done when

• A location is chosen and recorded in the standard.
• jvmLayout returns that location as the helper dir for multiplatform units, so the check runs for them again.
• Tests cover a helper outside the location (reported, with the right suggested dir) and inside it (not reported), in commonTest and in a target set such as jvmTest.
• Both READMEs describe the multiplatform helper location instead of the skip.