Add project documentation and language specification

[Copilot]

The repository had no usable README — only an unfinished, partially-commented-out language spec draft with a "PARSER NOT UP TO DATE" banner.

README.md

Replaced with a proper project-level README covering:

• What Zenith is and current implementation status
• Prerequisites (C++23 compiler, CMake 4.0)
• Build instructions (cmake -B build && cmake --build build)
• CLI usage with all supported flags (--target, --braces, --gc) and output artifacts
• How to run the GoogleTest suite
• Annotated project structure tree
• Roadmap checklist

docs/LANGUAGE_SPEC.md (new)

Full language reference extracted from the old README and restructured with a table of contents:

• Keywords, access modifiers, import directives
• Type system: primitives, freeobj, built-in class types
• Control flow and function syntax
• OOP: classes, inheritance, operator overloading, structs
• Metaprogramming: annotations and decorators (including how to define custom ones)
• Enum-based error handling with match
• Pipelines (WIP)
• Unique features table, EBNF grammar highlights, JVM interop and unsafe block examples

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