fix(api): tighten public surface before the 1.0 freeze

[davydog187]

API pre-freeze fixes: struct encoding, dead guard, option consistency

Plan: .agents/plans/A50-api-pre-freeze.md

These are the public-API sharp edges that would become breaking changes once 1.0 freezes the surface. Each must land before the freeze so the fix isn't a breaking change later.

Changes

1. Bare-struct encoding now raises (lib/lua/vm/value.ex). Structs are maps, so Lua.encode!(lua, %MyStruct{}) (and Lua.set!/3) silently encoded a Lua table carrying a "__struct__" key. A new %mod{} clause (before the is_map clause) raises Lua.RuntimeException with a message pointing at Map.from_struct/1 and noting the planned Lua.Encoder protocol (Ergonomic encode/decode for deflua (Lua.Encoder protocol + auto-marshalling) #341). Why before freeze: if struct encoding "works" accidentally today, adding the protocol in 1.1 becomes breaking instead of additive. Audit found no struct legitimately encoded in lib/ or test/ today.
2. Removed the dead is_mfa/1 guard (lib/lua/api.ex). A Luerl-era shim that always returned false, imported into every use Lua.API module. Removed the guard and the import entry; cleaned up the only other references (comments in a skipped test). Why before freeze: removing an exported guard after freeze is breaking.
3. Documented closure/tag tuples as unstable — is_lua_func/1, is_erl_func/1, and Lua.unwrap/1 now carry warnings that the :lua_closure / :compiled_closure / :native_func / :tref tuple internals are not stable API; use the guards. No tags were restructured.
4. :max_string_bytes accepts :infinity (lib/lua.ex, lib/lua/vm/state.ex, lib/lua/vm/limits.ex), uniform with :max_call_depth and :max_instructions. Enforcement is already :infinity-safe via Erlang term ordering (every number sorts below every atom); documented at the check site.
5. eval!/3 chunk clause accepts :source (lib/lua.ex). The string clause validated [decode:, source:] but the chunk clause only [decode:], so eval!(lua, chunk, source: "x") raised via Keyword.validate!. The chunk already carries its compiled-in source name, so :source is accepted but ignored (documented).
6. Documented call_function/3's error shape — added doctests showing {:error, reason, lua} where reason is the raw Lua-facing pcall error value (non-string values pass through verbatim; strings carry the source:line: prefix), distinct from the terminal-formatted call_function!/3.
7. Removed Lua.CompilerException's :state field — it was never populated (always nil). Moduledoc field list updated.
8. Reworded Lua.VM moduledoc from "Public API" to internal (kept readable for source/IEx readers); noted :reset_instructions is require-reentrancy machinery, not user API.
9. CHANGELOG Unreleased entries for items 1, 2, 4, 5, 7 in clearly-labeled subsections (breaking-before-freeze changes called out explicitly).

Verification

mix format --check-formatted   # clean
mix compile --warnings-as-errors  # clean (lua app, no warnings)
mix test                       # 2580 passed, 7 skipped, 30 excluded
mix test --only lua53          # 20 passed, 9 skipped
mix docs --warnings-as-errors  # generated, no warnings

Out of scope (intentional)

• The Lua.Encoder protocol itself (1.1, Ergonomic encode/decode for deflua (Lua.Encoder protocol + auto-marshalling) #341).
• Renaming closure tags / any tag-tuple restructuring.
• Docs-only fixes elsewhere (sibling PR A49).
• Version bump / release mechanics.

Plan: A50