fix(rsc): isolate plugin state per plugin instance

[hi-ogawa]

Description

Currently all plugin states are managed in module scope (as a reminiscence of my original prototype). These states should be isolated per plugin instance. This also helps splitting up more plugins from single file plugin.ts. Also helps fixing an issues like #575.

For now, let's do something mechanical by moving the state to RscPluginManager and just passing around manager. As a follow up, we could move some helpers to manager class such as assetURL method.

TODO

• test
  • build two different rsc apps programatically?

[Copilot]

Pull Request Overview

Refactors RSC plugin state management from module-level global variables to an isolated RscPluginManager class instance per plugin instance, enabling proper state isolation between multiple plugin instances.

Key changes:

• Introduces RscPluginManager class to encapsulate all plugin state
• Updates all plugin functions to accept and use the manager instance
• Adds test to verify independent plugin state isolation during multiple builds

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/plugin-rsc/src/plugin.ts	Removes global state variables, adds RscPluginManager class, and refactors all plugin functions to use manager instance
packages/plugin-rsc/e2e/starter.test.ts	Adds test case to verify plugin state isolation between multiple builds

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