docs: add missing stdlib modules and update language reference

Stdlib.md:
- Add Std::Task (thread spawn/join/sleep)
- Add Std::Channel (generic channels + wrappers)
- Add Std::Net (TCP sockets)
- Add Std::Json (parser/serializer)
- Add Std::Crypto (SHA-256, HMAC, Base64, random)
- Add Std::Fmt (formatting wrappers)
- Add Std::Math (Sqrt, Pow, Abs, Min, Max)
- Add String_FromFloat to String docs
- Update Future Modules list

LanguageRef.md:
- Add Concurrency section (threads + channels)
- Update Ownership with move semantics and own T

BuildAndTest.md:
- Add libssl-dev/openssl prerequisite note
- Mention -lcrypto auto-linking

docs/BuildAndTest.md

@@ -12,14 +12,16 @@ This guide covers building the Bux bootstrap compiler, creating projects, and ru
 
 On Debian/Ubuntu:
 ```bash
-sudo apt-get install nim gcc make
+sudo apt-get install nim gcc make libssl-dev
 ```
 
 On macOS:
 ```bash
-brew install nim gcc make
+brew install nim gcc make openssl
 ```
 
+> **Note:** The `Std::Crypto` module requires OpenSSL (`-lcrypto`). The build system links it automatically.
+
 ---
 
 ## Building the Compiler