4. Expected Sets as Interned IDs on a Shared Stack (11A6ce3)

• Error reporting records what was expected at the farthest failure. Every memoized call and every @error/#recover region used to start a fresh []string.
• Expectations are now compile-time IDs into a per-backend description table. Isolated regions are windows of one stack; only sets kept by the memo or a pending recovery are copied into an append-only arena.
• Effect: minilang 59 → 42 MB, ~128 → ~103 ms. Error messages are identical.