Add pending_generate + pending_regenerate flags to CodegenState
(mirror of chat.pending_send). Wire codegen: CodegenState as a
field on EditorState immediately after chat, with TDD coverage in
codegen_state_defaults_to_idle_react.
Plain-data types for the Code panel (Framework / CodegenPhase /
ChunkStatus / CodeGenProgress / ChunkProgress / AssetMeta /
CodegenState) in a new op-editor-core::codegen module. Keeps
op-editor-core wasm-clean; pipeline logic stays in op-codegen::ai
which already depends on this crate (acyclic).