errors: unify public runtime exceptions around Lua.RuntimeException

[davydog187]

Context

The public exception surface was inconsistent and leaked VM internals. Lua.call_function/3 returned raw Lua.VM.RuntimeError / Lua.VM.TypeError / Lua.VM.ArgumentError / … structs — inconsistently suffixed *Error and namespaced under the implementation detail Lua.VM. Since we're pre-1.0 (1.0.0-rc), this is the moment to settle the surface before it freezes.

What changed

The public exception surface is now exactly two types, both *Exception, neither under Lua.VM:

• Lua.CompilerException — compile/parse failures
• Lua.RuntimeException — every runtime failure

The five Lua.VM.*Error structs are now internal (@moduledoc false): they're wrapped into Lua.RuntimeException before crossing any API boundary, so they never surface raw. They stay under Lua.VM — the correct home for internal VM error types once they're not user-facing — which leaves the ~219 stdlib raise sites untouched (a low-risk change concentrated at the boundary).

Lua.RuntimeException grows two fields so callers can still discriminate:

• :kind — :error | :type | :argument | :assertion | :internal
• :value — the raised Lua value, exactly as pcall hands it back (§6.1)
• (:original still carries the internal VM struct for deep inspection)

Lua.call_function/3 now returns {:error, %Lua.RuntimeException{}, lua} uniformly. A side benefit: its :value now matches in-Lua pcall for string errors (both give "<eval>:1: boom"); previously it returned the un-prefixed "boom".

Files

• lib/lua/runtime_exception.ex — new :kind/:value; wrap clause derives them (value via existing ProtectedCall.error_value/1)
• lib/lua.ex — finish_call/2 wraps VM exceptions; updated call_function/3 docs
• lib/lua/vm/*_error.ex (×5) — @moduledoc false, prose preserved as comments
• CHANGELOG.md — public surface + call_function/3 return-type entries
• tests — rewrote the boundary pin, fixed one pcall-state test, added :kind/:value coverage

Verification

• mix test — 2598 passed (67 doctests, 56 properties), 7 skipped
• mix compile --warnings-as-errors clean; mix format clean
• End-to-end: call_function/3 returns %Lua.RuntimeException{kind:, value:, original:}; in-Lua pcall recovery unchanged ({false, "<eval>:1: boom"})

Note for review

Internal structs kept their *Error names (idiomatic Elixir for internal building blocks, and no longer user-facing). Renaming them to *Exception too is a mechanical follow-up if preferred.