Plugin Context Resolve

Summary

PluginContextResolveOptions::custom uses CustomField, and CustomField is intentionally backed by TypedMap instead of TypedDashMap.

Today this data is populated before the value is shared and then passed around as Arc<CustomField> for read-only access during resolve flows. Concurrent write access is not needed, so the extra sharding and synchronization overhead of TypedDashMap would be wasted here.

If plugin-context resolve grows features that need the same custom field to be written from multiple places concurrently, we can revisit this choice and switch CustomField back to TypedDashMap.

crates/rolldown_plugin/src/types/custom_field.rs
crates/rolldown_plugin/src/types/plugin_context_resolve_options.rs