refactor(start): streamline Rsbuild environment configuration

[SyMind]

Summary

This PR establishes a clearer Rsbuild configuration workflow for TanStack Start and fixes #7827.

flowchart LR
    D["Framework defaults<br/>Applied only when unset"]
    U["User config<br/>Native root and environment config"]
    E["Framework enforced config<br/>Applied last"]
    C{"Conflicts with<br/>an enforced value?"}
    W["Print warning with<br/>overridden paths"]
    F["Final Rsbuild config"]

    D -->|"lowest priority"| U
    U -->|"overrides defaults"| E
    E --> C
    C -->|"yes"| W
    C -->|"no"| F
    W --> F

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The resulting precedence is:

framework defaults -> user config -> framework enforced config

• Framework defaults provide working conventions but remain user-configurable.
• User config uses native Rsbuild root and environment configuration.
• Framework-enforced config contains values required by the TanStack Start runtime. Conflicts produce a warning.

Notable changes

• Stop writing assetPrefix into the client environment, allowing root configuration and downstream plugins to work correctly.
• Resolve development and production asset bases from dev.assetPrefix and output.assetPrefix.
• Remove the Start-specific rsbuild.client.output and custom environment override abstractions.
• Derive the manifest script format from the resolved client output.module.
• Apply enforced entries, defines, and aliases consistently across client, SSR, and provider environments.

Testing

• 519 unit tests passed.
• Type tests passed with TypeScript 5.6–7.0.
• ESLint passed.

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[SyMind]

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Codex Review: Didn't find any major issues. Nice work!

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