Addresses the review question on #8413: is Arc<dyn TsconfigFinder> necessary, or can Resolver be used directly?

Why Resolver cannot be used directly in RawTransformOptions:

RawTransformOptions lives in rolldown_common. rolldown_resolver (which owns Resolver<Fs>) already depends on rolldown_common, making the reverse direction a circular dependency.

Alternatives evaluated:

• Arc<ResolverGeneric<Fs>> from oxc_resolver — would require making RawTransformOptions<Fs> generic, propagating through TransformOptionsInner, TransformOptions, and NormalizedBundlerOptions; or adding rolldown_fs as a rolldown_common dependency for the OsFileSystem default. Both add more coupling than the current trait.
• Arc<dyn Fn(&Path) -> ...> — avoids a named type but is less readable; same dynamic dispatch underneath.
• Thread Resolver as a parameter to options_for_file — options_for_file is defined in rolldown_common, so it still requires an abstraction to reference the rolldown_resolver type there.

TsconfigFinder is the standard Rust dependency-inversion pattern for this constraint. No code changes made; reply left on the review thread.

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