refactor: extract op-codegen crate

Relocate codegen.rs + codegen_targets.rs out of openpencil-shell-core
into a dedicated op-codegen crate. The code generators only depend on
jian-ops-schema + op-editor-core, so they form a clean leaf crate.
codegen.rs becomes the crate's lib.rs; crate::codegen:: paths in
codegen_targets.rs collapse to crate::. Pure relocation, no behaviour
change; tests move with their code.

crates/op-codegen/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "op-codegen"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+description = "OpenPencil code generators — emit HTML/Vue/Svelte/React/Flutter/SwiftUI/Compose/RN from a PenDocument"
+
+[lib]
+name = "op_codegen"
+path = "src/lib.rs"
+
+[dependencies]
+jian-ops-schema = { path = "../../vendor/jian/crates/jian-ops-schema" }
+op-editor-core = { path = "../op-editor-core" }

crates/op-codegen/src/codegen_targets.rs

@@ -1,13 +1,13 @@
 //! Per-framework markup generators — the React / Vue / Svelte / HTML
 //! / Flutter / SwiftUI / Compose / React Native targets carved off
-//! `codegen.rs` to keep both files under the 800-line cap.
+//! `lib.rs` to keep both files under the 800-line cap.
 //!
 //! All generators consume the canonical `jian_ops_schema::PenDocument`
-//! and walk `PenNode` through the flat-view accessors in the parent
-//! `codegen` module (`node_origin` / `node_size` / `node_fill_css` /
+//! and walk `PenNode` through the flat-view accessors in the crate
+//! root (`node_origin` / `node_size` / `node_fill_css` /
 //! …) so the per-target emit logic stays variant-agnostic.
 
-use crate::codegen::{
+use crate::{
     color_to_css, fmt_num, html_escape, node_children, node_corner_radius, node_fill_css,
     node_hidden, node_is_ellipse, node_is_text, node_origin, node_rotation_deg, node_size,
     node_stroke_css, node_text, parse_color, root_nodes, Codegen, CssVariables,

crates/op-codegen/src/lib.rs

@@ -12,6 +12,8 @@
 //! node-accessor helpers, and the test suite; per-framework markup
 //! generators live in the sibling `codegen_targets` module.
 
+mod codegen_targets;
+
 pub use crate::codegen_targets::{
     Compose, Flutter, Html, React, ReactNative, Svelte, SwiftUi, Vue,
 };

crates/openpencil-shell-core/src/lib.rs

@@ -19,8 +19,6 @@
 pub mod agent_settings_state;
 pub mod chat_models;
 pub mod chat_provider;
-pub mod codegen;
-mod codegen_targets;
 pub mod figma;
 // Phase 3 strangler reorg: i18n extracted into the op-i18n crate. Re-exported
 // as `i18n` so `crate::i18n::translate` / `crate::i18n::Locale` paths still resolve.