feat: Add ZRTL standard library plugins

Add a library of ZRTL plugins providing common language features:

- zrtl_io: I/O operations (print, input, formatting) using ZRTL string format
- zrtl_fs: File system operations (read/write files, directories, paths)
- zrtl_time: Time operations (timestamps, sleep, instant timing, formatting)
- zrtl_env: Environment (env vars, args, process info, OS detection)
- zrtl_thread: Threading (spawn/join, atomics, mutexes)
- zrtl_net: Networking (TCP client/server, UDP)

All plugins:
- Use ZRTL SDK types (StringPtr, ArrayPtr) for C ABI compatibility
- Export symbols using $Module$function naming convention
- Include comprehensive tests (26 tests total)
- Compile to both cdylib and rlib for flexibility

Author: darmie

plugins/Cargo.toml

@@ -0,0 +1,18 @@
+[workspace]
+resolver = "2"
+members = [
+    "zrtl_io",
+    "zrtl_fs",
+    "zrtl_thread",
+    "zrtl_net",
+    "zrtl_time",
+    "zrtl_env",
+]
+
+[workspace.dependencies]
+# Core ZRTL types
+zrtl = { path = "../sdk/zrtl" }
+
+# Common dependencies
+thiserror = "2.0"
+log = "0.4"

plugins/zrtl_env/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "zrtl_env"
+version = "0.1.0"
+edition = "2021"
+description = "ZRTL Environment plugin - env vars, args, process"
+license = "Apache-2.0"
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+zrtl = { path = "../../sdk/zrtl" }