runner/json_v0_bridge: split into module dir with ast.rs, lowering.rs, lexer.rs, mod.rs. Keep public API (parse_json_v0_to_module, parse_source_v0_to_module, maybe_dump_mir) unchanged; introduce BridgeEnv/VarScope in lowering; move tiny lexer/parser. Build + PyVM Stage-2 smokes PASS.

src/runner/json_v0_bridge/mod.rs

@@ -0,0 +1,24 @@
+mod ast;
+mod lowering;
+mod lexer;
+
+use ast::ProgramV0;
+use lowering::lower_program;
+
+pub fn parse_json_v0_to_module(json: &str) -> Result<crate::mir::MirModule, String> {
+    let prog: ProgramV0 = serde_json::from_str(json).map_err(|e| format!("invalid JSON v0: {}", e))?;
+    if prog.version != 0 || prog.kind != "Program" {
+        return Err("unsupported IR: expected {version:0, kind:\"Program\"}".into());
+    }
+    lower_program(prog)
+}
+
+pub fn parse_source_v0_to_module(input: &str) -> Result<crate::mir::MirModule, String> {
+    let json = lexer::parse_source_v0_to_json(input)?;
+    if std::env::var("NYASH_DUMP_JSON_IR").ok().as_deref() == Some("1") {
+        println!("{}", json);
+    }
+    parse_json_v0_to_module(&json)
+}
+
+pub fn maybe_dump_mir(module: &crate::mir::MirModule) { lowering::maybe_dump_mir(module) }