gnd: Validate event/call handler signatures against ABI

Add validation that event handler event signatures and call handler
function signatures match entries in the source ABI. This catches
errors like referencing a nonexistent event or using wrong parameter
types early, during codegen rather than at deploy time.

The matching logic mirrors graph-node's runtime matching:
- Event signatures support both indexed and non-indexed forms
- Non-indexed fallback only matches if a single event variant exists
- Function signatures use canonical form: name(type1,type2,...)

Changes:
- Add `event` field to EventHandler (Solidity event signature)
- Replace `call_handlers: Vec<String>` with `Vec<CallHandler>` struct
  containing both the function signature and handler name
- Add validate_handler_signatures() wired into both codegen and build
- Update test ABI fixtures to include actual events/functions

Author: lutter

gnd/src/commands/codegen.rs

@@ -657,7 +657,17 @@ type MyEntity @entity(immutable: true) {
 "#;
         fs::write(project_dir.join("schema.graphql"), schema_content).unwrap();
         fs::write(project_dir.join("mapping.ts"), "").unwrap();
-        fs::write(project_dir.join("TestContract.json"), "[]").unwrap();
+        let abi = r#"[{
+            "type": "event",
+            "name": "Transfer",
+            "anonymous": false,
+            "inputs": [
+                {"name": "from", "type": "address", "indexed": true},
+                {"name": "to", "type": "address", "indexed": true},
+                {"name": "value", "type": "uint256", "indexed": false}
+            ]
+        }]"#;
+        fs::write(project_dir.join("TestContract.json"), abi).unwrap();
 
         // Run codegen
         let opt = CodegenOpt {

gnd/src/manifest.rs

@@ -70,8 +70,8 @@ pub struct DataSource {
     pub source_abi: Option<String>,
     /// Event handlers from the mapping.
     pub event_handlers: Vec<EventHandler>,
-    /// Call handler names from the mapping.
-    pub call_handlers: Vec<String>,
+    /// Call handlers from the mapping.
+    pub call_handlers: Vec<CallHandler>,
     /// Block handlers from the mapping (with filter info for validation).
     pub block_handlers: Vec<BlockHandler>,
 }
@@ -96,8 +96,8 @@ pub struct Template {
     pub source_abi: Option<String>,
     /// Event handlers from the mapping.
     pub event_handlers: Vec<EventHandler>,
-    /// Call handler names from the mapping.
-    pub call_handlers: Vec<String>,
+    /// Call handlers from the mapping.
+    pub call_handlers: Vec<CallHandler>,
     /// Block handlers from the mapping (with filter info for validation).
     pub block_handlers: Vec<BlockHandler>,
 }
@@ -119,13 +119,23 @@ pub struct Abi {
 /// An event handler with metadata needed for validation.
 #[derive(Debug)]
 pub struct EventHandler {
+    /// The Solidity event signature, e.g. `Transfer(address,address,uint256)`.
+    pub event: String,
     pub handler: String,
     /// Whether this handler requires transaction receipts.
     pub receipt: bool,
     /// Whether this handler has eth call declarations.
     pub has_call_decls: bool,
 }
 
+/// A call handler with metadata needed for validation.
+#[derive(Debug)]
+pub struct CallHandler {
+    /// The Solidity function signature, e.g. `approve(address,uint256)`.
+    pub function: String,
+    pub handler: String,
+}
+
 /// A block handler with filter info needed for validation.
 #[derive(Debug)]
 pub struct BlockHandler {
@@ -223,6 +233,7 @@ fn convert_data_source(ds: GraphUnresolvedDS<Chain>) -> DataSource {
                 .event_handlers
                 .iter()
                 .map(|h| EventHandler {
+                    event: h.event.clone(),
                     handler: h.handler.clone(),
                     receipt: h.receipt,
                     has_call_decls: !h.calls.raw_decls.is_empty(),
@@ -232,7 +243,10 @@ fn convert_data_source(ds: GraphUnresolvedDS<Chain>) -> DataSource {
                 .mapping
                 .call_handlers
                 .iter()
-                .map(|h| h.handler.clone())
+                .map(|h| CallHandler {
+                    function: h.function.clone(),
+                    handler: h.handler.clone(),
+                })
                 .collect(),
             block_handlers: eth
                 .mapping
@@ -320,6 +334,7 @@ fn convert_template(t: GraphUnresolvedDST<Chain>) -> Template {
                 .event_handlers
                 .iter()
                 .map(|h| EventHandler {
+                    event: h.event.clone(),
                     handler: h.handler.clone(),
                     receipt: h.receipt,
                     has_call_decls: !h.calls.raw_decls.is_empty(),
@@ -329,7 +344,10 @@ fn convert_template(t: GraphUnresolvedDST<Chain>) -> Template {
                 .mapping
                 .call_handlers
                 .iter()
-                .map(|h| h.handler.clone())
+                .map(|h| CallHandler {
+                    function: h.function.clone(),
+                    handler: h.handler.clone(),
+                })
                 .collect(),
             block_handlers: eth
                 .mapping