feat(node): add image ID storage and retrieval

docs/plans/2026-01-09-image-id-storage-design.md

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+# Image ID Storage Design
+
+Store RISC0 image IDs (state and recursive) in node storage and expose them via Node methods.
+
+## Background
+
+Currently, image IDs are computed on-demand from ELF code during recursive proof generation but not persisted. This design adds persistent storage and retrieval methods.
+
+## Requirements
+
+- Store state and recursive image IDs as `[u8; 32]`
+- Compute and store at node startup when ELF codes are provided
+- Support both Db (SQLite) and Memory storage backends
+- Expose via two methods: `get_state_image_id()` and `get_recursive_image_id()`
+
+## Design
+
+### Database Schema
+
+New file: `node/sql/create/image_ids.sql`
+
+```sql
+CREATE TABLE IF NOT EXISTS image_ids (
+  id INTEGER PRIMARY KEY,
+  state_image_id BLOB NOT NULL,
+  recursive_image_id BLOB NOT NULL
+)
+```
+
+Single row table holding both image IDs.
+
+### Storage Layer
+
+**`node/src/db.rs`** - Add methods:
+
+```rust
+pub async fn set_image_ids(&self, state: [u8; 32], recursive: [u8; 32])
+pub async fn get_state_image_id(&self) -> Option<[u8; 32]>
+pub async fn get_recursive_image_id(&self) -> Option<[u8; 32]>
+```
+
+**`node/src/memory.rs`** - Add fields to `MemoryInner`:
+
+```rust
+state_image_id: Option<[u8; 32]>,
+recursive_image_id: Option<[u8; 32]>,
+```
+
+With matching setter and getter methods on `Memory`.
+
+**`node/src/storage.rs`** - Add delegation on `Storage` enum:
+
+```rust
+pub async fn set_image_ids(&self, state: [u8; 32], recursive: [u8; 32])
+pub async fn get_state_image_id(&self) -> Option<[u8; 32]>
+pub async fn get_recursive_image_id(&self) -> Option<[u8; 32]>
+```
+
+### Node API
+
+**`node/src/lib.rs`** - Add methods on `Node`:
+
+```rust
+pub async fn get_state_image_id(&self) -> Option<[u8; 32]> {
+    self.storage.get_state_image_id().await
+}
+
+pub async fn get_recursive_image_id(&self) -> Option<[u8; 32]> {
+    self.storage.get_recursive_image_id().await
+}
+```
+
+### Initialization
+
+In the ZK publisher flow (`run_zk_publisher_stream`), at startup when `ElfCode` is available:
+
+```rust
+let state_image_id: [u8; 32] = risc0_zkvm::compute_image_id(&elf_code.state_elf_code)?.into();
+let recursive_image_id: [u8; 32] = risc0_zkvm::compute_image_id(&elf_code.recursive_elf_code)?.into();
+storage.set_image_ids(state_image_id, recursive_image_id).await;
+```
+
+This runs once at startup before the main processing loop.
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `node/sql/create/image_ids.sql` | New table schema |
+| `node/src/db.rs` | Add set/get methods for image IDs |
+| `node/src/memory.rs` | Add fields and methods to MemoryInner/Memory |
+| `node/src/storage.rs` | Add delegation methods on Storage enum |
+| `node/src/lib.rs` | Add Node methods and initialization logic |
+
+## Not In Scope
+
+- HTTP endpoints for image IDs (can be added later if needed)
+- Storing full ELF code bytes
+- Changes to proof storage or retrieval