refactor(codegen): extract Codecs as an injected component (layered DI, step 1)

First step of converting codegen passes to DI'd classes. The JSON-codec + event/
web-dispatch emitters are a clean leaf (no recursion into expression/statement
gen), so they become `class JsonCodecs implements Codecs`, injected into
XiCodegen and threaded into genAll. Shared expr-building helpers (jsonEncodeExpr/
jsonDecodeExpr/arrElemCtype) stay free. Acyclic graph confirmed: expr passes
never call statements, so ExpressionGen <- StatementGen <- orchestrator is the
next steps. Self-hosting fixpoint byte-identical.

Author: code-by-sia

compiler/contracts/codecs.xi

@@ -0,0 +1,9 @@
+// Codecs — JSON (de)serialization + event/web dispatch generation. A leaf
+// codegen component (no recursion into expression/statement gen). Implemented by
+// JsonCodecs (impl/codegen/codecs.xi); resolved by genAll.
+interface Codecs {
+    mapper genEventCodecs(prog: Program) -> String
+    mapper genEventFwd(prog: Program) -> String
+    mapper genEventDispatch(prog: Program) -> String
+    mapper genWebDispatch(prog: Program) -> String
+}