Fork a self-contained Python parser and portable bytecode frontend

[deltathedumb]

Removes the final source-parsing dependency on CPython for the first executable subset.

• forks asmpython's Python-authored lexer, parser, AST nodes, and diagnostics into PortaPy
• removes executable compiler-extension hooks and keeps standard Python syntax only
• records the pinned upstream parser source
• makes future parser synchronization manual and reviewed
• adds a direct PortaPy-AST-to-bytecode lowerer with no host ast import
• executes literals, names, arithmetic, assignment, functions, calls, and returns through the existing VM
• fails precisely for unsupported syntax rather than silently falling back to CPython

This is the first self-contained source path. More AST nodes still need to move into the portable frontend before the bootstrap host-ast frontend can be deleted.