[codex] modernjs-v3 RSC+MF SSR support adjustments

[ScriptedAlchemy]

Summary

This patch aligns @module-federation/modern-js-v3 behavior with RSC + Module Federation SSR requirements by adding targeted guards in the Modern.js v3 CLI plugins.

For apps running with server.rsc: true, the current defaults and SSR wiring can produce unstable server-bundle/runtime behavior in strict MF+RSC runs. The changes in this PR are intentionally small and scoped to RSC mode to avoid broad behavioral drift for non-RSC applications.

Problem

In strict RSC+MF integration scenarios:

• Forcing server target behavior without preserving RSC resolution conditions can cause server package resolution/runtime failures.
• Keeping source.enableAsyncEntry defaulted to true for RSC apps leads to unresolved server-bundle expectations (Cannot find server bundle-style failures in matrix runs).
• Forcing ssr.distOutputDir in RSC mode can conflict with RSC server output assumptions and contributes to mismatched server artifact lookup.

Root Cause

modernjs-v3 plugin defaults and SSR pathing were not conditioned on server.rsc for key MF+SSR points:

• Server chain target coercion did not also preserve react-server condition resolution.
• Async entry default remained globally true when not explicitly configured by users.
• SSR distOutputDir auto-assignment always ran when SSR was enabled.

Fix

1) Preserve RSC server resolution when using async-node

In packages/modernjs-v3/src/cli/ssrPlugin.ts:

• detect enableRsc from modernjsConfig?.server?.rsc
• keep chain.target('async-node') for server MF
• add chain.resolve.conditionNames.add('react-server') when RSC is enabled

2) Default enableAsyncEntry to false for RSC apps

In packages/modernjs-v3/src/cli/configPlugin.ts:

• detect enableRsc
• change source default to:
  • modernjsConfig.source?.enableAsyncEntry ?? !enableRsc

This preserves explicit user config while making RSC-safe behavior the default.

3) Skip forced ssr.distOutputDir in RSC mode

In packages/modernjs-v3/src/cli/configPlugin.ts:

• gate SSR dist output injection to non-RSC mode:
  • else if (enableSSR && !enableRsc)

User Impact

For RSC+MF users, this reduces server runtime/bundle resolution instability and aligns defaults with strict RSC expectations. Non-RSC behavior remains unchanged.

Validation

Executed:

• pnpm -C packages/modernjs-v3 exec vitest run src/cli/configPlugin.spec.ts src/server/fileCache.spec.ts src/server/staticMiddleware.spec.ts

Result:

• 3 test files passed
• 15 tests passed

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Bundle Size Report

1 package(s) changed, 40 unchanged.

Package	Total dist	Delta	ESM gzip	Delta
modern-js-plugin-v3	187.7 kB	+1.5 kB (+0.8%)	900 B	no change

Total dist: 6.89 MB (+1.5 kB (+0.0%))
Total ESM gzip: 138.6 kB (no change)

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