Completely remove inline-Python (::py::) from the language

[marsninja]

Summary

Completely remove the inline-Python (::py:: ... ::py::) feature from the Jac language. The native backend already strips ::py:: blocks (they cannot be lowered to LLVM IR), which makes inline Python a source of divergence between the bytecode/Python pathway and the native pathway. Rather than implement it in native, we want to drop the feature from the language entirely.

Scope

• Remove ::py:: grammar/lexer/parser support
• Remove the corresponding AST node(s) for inline Python blocks
• Remove handling across all compiler passes (Python codegen, native pass strip logic, type-checking, etc.)
• Remove/migrate any usages of ::py:: in the standard library, examples, and tests
• Update documentation and language reference to drop the construct
• Provide a clear migration path / error message for code that still uses ::py:: (e.g. "inline Python is no longer supported; use a Python import instead")

Motivation

• Inline Python is fundamentally unsupportable in the native (LLVM) backend, creating a permanent feature gap between pathways.
• It bypasses Jac's type system and tooling.
• Python interop is already achievable via standard import mechanisms, making ::py:: redundant.

Acceptance criteria

• No ::py:: syntax is accepted by the parser; encountering it produces a clear, actionable error.
• All references to inline Python are removed from grammar, AST, passes, stdlib, examples, tests, and docs.
• Full test suite passes after removal/migration.