Move RaindexRegistry core logic from js_api to common

[findolor]

Summary

DotrainRegistry in crates/js_api/src/registry.rs (~1,639 lines) contains platform-agnostic business logic trapped in the WASM-only crate. No other Rust crate can depend on js_api, so CLI and integration tests can't reuse registry functionality. Rename from DotrainRegistry to RaindexRegistry for consistency with Raindex* naming convention.

What needs to move

Core logic (~90% of the file):

• Registry file format parsing (parse_registry_content)
• HTTP fetching of registry, settings, and order files
• Registry format validation
• Order metadata extraction (name/description from dotrain configs)
• Error type: DotrainRegistryError → RaindexRegistryError

Key renames

• DotrainRegistry → RaindexRegistry
• DotrainRegistryError → RaindexRegistryError

What stays in js_api

Thin WASM wrapper (~10%):

• #[wasm_bindgen] struct decorator and #[wasm_export] method attributes
• js_sys::Function callback parameter for state updates
• JsValue error conversions
• Wrapper methods that delegate to the core implementation

Proposed approach

1. Create common::registry module with the core RaindexRegistry struct and logic
2. Keep js_api::registry as a thin wrapper that re-exports/wraps the core type with WASM annotations
3. Replace js_sys::Function callbacks with a Rust trait or generic callback

Benefits

• CLI can implement native registry support
• Integration tests can test registry logic without WASM
• Reduces coupling between business logic and platform bindings
• Consistent Raindex* naming across the codebase