feat: Add enhanced debug logging for plugin VM communication

- Add VM→Plugin and Plugin→VM boundary logging for method calls
- Enhanced debug output for tracking plugin invocation flow
- Added return value type/instance logging for better debugging
- Improved plugin communication traceability

This helps debug issues like the recent 45-day HTTP response debugging saga.
All logging is controlled by NYASH_DEBUG_PLUGIN=1 environment variable.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/plugin_contract_net_ids.rs

@@ -0,0 +1,39 @@
+#![cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
+
+use nyash_rust::parser::NyashParser;
+use nyash_rust::runtime::plugin_loader_v2::{init_global_loader_v2, get_global_loader_v2};
+use nyash_rust::runtime::box_registry::get_global_registry;
+use nyash_rust::runtime::PluginConfig;
+
+fn try_init_plugins() -> bool {
+    if !std::path::Path::new("nyash.toml").exists() { return false; }
+    if let Err(e) = init_global_loader_v2("nyash.toml") { eprintln!("init failed: {:?}", e); return false; }
+    let loader = get_global_loader_v2();
+    let loader = loader.read().unwrap();
+    if let Some(conf) = &loader.config {
+        let mut map = std::collections::HashMap::new();
+        for (lib, def) in &conf.libraries { for b in &def.boxes { map.insert(b.clone(), lib.clone()); } }
+        get_global_registry().apply_plugin_config(&PluginConfig { plugins: map });
+        true
+    } else { false }
+}
+
+/// Minimal ABI sanity check: HttpRequestBox.path=1, readBody=2
+#[test]
+fn plugin_contract_http_request_ids_sanity() {
+    if !try_init_plugins() { return; }
+    // Exercise HttpRequestBox.path/readBody on a birthed request (no server needed)
+    let code = r#"
+local req, p, b
+req = new HttpRequestBox()
+p = req.path()
+b = req.readBody()
+p + ":" + b
+"#;
+    let ast = NyashParser::parse_from_string(code).expect("parse failed");
+    let mut interpreter = nyash_rust::interpreter::NyashInterpreter::new();
+    let result = interpreter.execute(ast).expect("exec failed");
+    // Default path="", body="" for birthed request
+    assert_eq!(result.to_string_box().value, ":");
+}
+