codegen: type-driven dispatch for + between Int idents

Closes the known limitation from the self-host commit: `fn add(a:Int,
b:Int) { a + b }` now compiles to integer addition, not string concat.
Previously the codegen heuristic guessed string concat whenever both
operands were Idents with no literal anchor.

Mechanism
- parser captures the leading type identifier from `let x: T = ...`
  bindings (new "type" field on Let) and from function parameter
  annotations (new "type" field on each param).
- codegen maintains a per-function int-name set in process state via
  state_set("__int_names", csv). cg_fn seeds it from typed parameters;
  cg_stmt extends it from typed `let` bindings and from `let x = <Int
  literal>` (literal inference).
- BinOp Plus: when both sides are Idents and both names are in the
  int-name set, emit arithmetic; otherwise the existing literal-anchor
  heuristic applies, with string concat as the fallback.

This is the first compiler change made entirely through the self-
hosting workflow — no Python bootstrap. Edit el source, run existing
elc on elc-combined.el, cc the output, test. Closure holds at the
new binary.

Tests
- add(40, 2) → 42
- count_to(10) → 45 (let i: Int / let total: Int rebinding)
- Regression suite (tiny/implret/whiletest/lextest) unchanged.

dist/platform/elc updated; .prev preserved.

el-compiler/src/codegen.el

@@ -170,6 +170,23 @@ fn cg_expr(expr: Map<String, Any>) -> String {
                 let op_c: String = binop_to_c(op)
                 return "(" + left_c + " " + op_c + " " + right_c + ")"
             }
+            // Type-driven dispatch: if both sides are Idents declared
+            // with type Int (parameters annotated `: Int` or let bindings
+            // annotated `: Int`), this is arithmetic, not concat. The
+            // current-function int-name set is maintained by cg_fn /
+            // cg_stmt via state_set("__int_names", csv).
+            if left_kind == "Ident" {
+                if right_kind == "Ident" {
+                    let lname: String = left["name"]
+                    let rname: String = right["name"]
+                    if is_int_name(lname) {
+                        if is_int_name(rname) {
+                            let op_c: String = binop_to_c(op)
+                            return "(" + left_c + " " + op_c + " " + right_c + ")"
+                        }
+                    }
+                }
+            }
             if left_kind == "Call" {
                 return "el_str_concat(" + left_c + ", " + right_c + ")"
             }
@@ -188,8 +205,8 @@ fn cg_expr(expr: Map<String, Any>) -> String {
                     return "el_str_concat(" + left_c + ", " + right_c + ")"
                 }
             }
-            // Ident + Ident or Ident + unknown — assume string concat
-            // (This is the ambiguous case: El uses + for both string and integer ops)
+            // Ident + Ident or Ident + unknown without int-typed evidence —
+            // fall back to string concat (the historical heuristic).
             if left_kind == "Ident" {
                 return "el_str_concat(" + left_c + ", " + right_c + ")"
             }
@@ -413,6 +430,17 @@ fn cg_stmt(stmt: Map<String, Any>, indent: String, declared: [String]) -> [Strin
         let name: String = stmt["name"]
         let val = stmt["value"]
         let val_c: String = cg_expr(val)
+        // If the binding is annotated `: Int` and val is an Int literal,
+        // register `name` in the per-function int-name set so that later
+        // `name + ...` dispatches to arithmetic, not concat.
+        let ltype: String = stmt["type"]
+        if str_eq(ltype, "Int") {
+            add_int_name(name)
+        }
+        let vk: String = val["expr"]
+        if str_eq(vk, "Int") {
+            add_int_name(name)
+        }
         if list_contains(declared, name) {
             emit_line(indent + name + " = " + val_c + ";")
             return declared
@@ -627,6 +655,41 @@ fn transform_implicit_return(body: [Map<String, Any>]) -> [Map<String, Any>] {
     body
 }
 
+// Test whether `name` is currently registered as an Int-typed identifier
+// for the function being codegened. The set is maintained as a comma-
+// bounded CSV in process state; cg_fn seeds it from typed parameters,
+// cg_stmt extends it from typed `let` bindings.
+fn is_int_name(name: String) -> Bool {
+    let csv: String = state_get("__int_names")
+    if str_eq(csv, "") { return false }
+    return str_contains(csv, "," + name + ",")
+}
+
+fn add_int_name(name: String) -> Bool {
+    let csv: String = state_get("__int_names")
+    if str_eq(csv, "") { csv = "," }
+    let key: String = "," + name + ","
+    if str_contains(csv, key) { return true }
+    state_set("__int_names", csv + name + ",")
+    return true
+}
+
+fn build_int_names_for_params(params: [Map<String, Any>]) -> Bool {
+    state_set("__int_names", ",")
+    let np: Int = native_list_len(params)
+    let pi = 0
+    while pi < np {
+        let param = native_list_get(params, pi)
+        let pname: String = param["name"]
+        let ptype: String = param["type"]
+        if str_eq(ptype, "Int") {
+            add_int_name(pname)
+        }
+        let pi = pi + 1
+    }
+    return true
+}
+
 fn cg_fn(stmt: Map<String, Any>) -> Void {
     let fn_name: String = stmt["name"]
     // Skip El's `fn main()` — C provides its own main() for top-level stmts
@@ -636,6 +699,9 @@ fn cg_fn(stmt: Map<String, Any>) -> Void {
     let body = stmt["body"]
     let ret_type: String = stmt["ret_type"]
     let params_c: String = params_to_c(params)
+    // Seed the per-function int-name set so the `+` codegen can dispatch
+    // arithmetic vs concat on type-annotated identifiers.
+    build_int_names_for_params(params)
     emit_line("el_val_t " + fn_name + "(" + params_c + ") {")
     // Seed declared with parameter names so reassignment works
     let decl = native_list_empty()