[Enhancement] - Error Numbers / Categories for runtime failures

[DerekCorniello]

Summary

Companion to muxlang/mux-compiler#314, which proposes numbered/categorized
compiler diagnostics and explicitly notes a similar issue is needed here. That
scheme reserves a runtime band:

E0600  runtime

Runtime failures today are bare panic strings with no stable identifier, so
they cannot be documented, searched, or matched on by tooling the way a coded
diagnostic can.

Scope

Everything the runtime aborts or panics on from a compiled Mux program:

• index out of bounds (list read/write after normalization)
• map key not present
• division / modulo by zero
• integer overflow where checked
• where-clause constraint violation at runtime (the documented fallback when
  a violation is not provable at compile time)
• allocation failure, and other internal invariants

Open design questions

• Numbering ownership. The compiler and runtime are versioned and released
  independently, so a shared code space needs an owner. Options: reserve a
  fixed band per repo (E0600-E0699 here) and keep the registry in mux-context,
  or namespace runtime codes separately.
• Where the registry lives. A machine-readable list (one file, checked in
  CI as the single source of truth) beats codes scattered across panic sites,
  and gives the docs site something to generate a diagnostics index from.
• Message format. Whether the code prefixes the panic text, and whether a
  runtime panic prints a doc URL the way rustc's --explain does.

Depends on

The category layout in muxlang/mux-compiler#314 should be settled first, since
this is one band inside it.