Merge selfhosting-dev into main (Core-13 pure CI/tests + LLVM bridge) (#126)

* WIP: sync before merging origin/main

* fix: unify using/module + build CLI; add missing helper in runner; build passes; core smokes green; jit any.len string now returns 3

* Apply local changes after merging main; keep docs/phase-15 removed per main; add phase-15.1 docs and tests

* Remove legacy docs/phase-15/README.md to align with main

* integration: add Core-13 pure CI, tests, and minimal LLVM execute bridge (no docs) (#125)

Co-authored-by: Tomoaki <tomoaki@example.com>

---------

Co-authored-by: Selfhosting Dev <selfhost@example.invalid>
Co-authored-by: Tomoaki <tomoaki@example.com>

src/tests/mir_pure_e2e_vm.rs

@@ -0,0 +1,30 @@
+#[cfg(test)]
+mod tests {
+    use crate::parser::NyashParser;
+    use crate::backend::VM;
+
+    #[test]
+    fn vm_exec_new_string_length_under_pure_mode() {
+        // Enable Core-13 pure mode
+        std::env::set_var("NYASH_MIR_CORE13_PURE", "1");
+
+        // Nyash code: return (new StringBox("Hello")).length()
+        let code = r#"
+return (new StringBox("Hello")).length()
+"#;
+
+        // Parse -> MIR -> VM execute
+        let ast = NyashParser::parse_from_string(code).expect("parse");
+        let mut compiler = crate::mir::MirCompiler::new();
+        let result = compiler.compile(ast).expect("compile");
+
+        let mut vm = VM::new();
+        let out = vm.execute_module(&result.module).expect("vm exec");
+        // Expect 5 as string (to_string_box) for convenience
+        assert_eq!(out.to_string_box().value, "5");
+
+        // Cleanup
+        std::env::remove_var("NYASH_MIR_CORE13_PURE");
+    }
+}
+