hal/bytecode-vm

Rust HAL staged bytecode VM (milestones 1-5: synchronous core through persistent artifacts)

Rust-only implementation contract for the experimental, feature-gated bytecode VM: execution model, instruction set, persistent program representation, validation, lexical slots, stackless function calls, exception handling, globals and namespaces, primitive dispatch, diagnostics, and conformance. It does not require Java or another HAL runtime to implement a stack VM, this opcode set, numeric slots, frames, handlers, or this compiler. Portable HAL language behavior remains owned by the shared language specifications and corpora.