chore(rvm): add debug-mode invariant assertions (#737)

Encode VM stack/context/register lifecycle invariants as
debug_assert!s. Zero cost in release; surfaces violations during
debug-mode tests and CI.

Invariants covered:
- reset_execution_state postcondition: all stacks empty, registers
  resized to base and Undefined, rule_cache reset, pc/executed
  counters zeroed, builtins_cache cleared, execution_state Ready.
- Per-opcode invariant check (assert_vm_invariants) invoked at the
  top of run_stackless_loop and jump_to iterations: state is
  Ready/Running, registers non-empty, rule_cache sized to program,
  execution stack bounded by a debug-only sanity ceiling
  (DEBUG_MAX_EXECUTION_STACK_DEPTH = 4096; not a production limit).
- resume() precondition: execution_state is Suspended.
- execute_suspendable_entry precondition: clean state (callers reset
  immediately before).
- Rule finalize: call_rule_stack pop matches the finalized rule_index.
- IterationState::advance: Single iterator not advanced past
  consumption, Array index not at usize::MAX before saturating_add.

All assertions are gated by #[cfg(debug_assertions)] (directly or via
debug_assert!) so release builds are unaffected.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/rvm/vm/comprehension.rs

@@ -528,7 +528,6 @@ impl RegoVM {
             Ok(false)
         }
     }
-
     pub(super) fn handle_comprehension_condition_failure_suspendable(&mut self) -> Result<bool> {
         if let Some(mut frame) = self.execution_stack.pop() {
             let handled = if let &mut FrameKind::Comprehension {
@@ -606,6 +605,9 @@ impl RegoVM {
     }
 
     fn execute_comprehension_end_run_to_completion(&mut self) -> Result<()> {
+        // `ComprehensionEnd` is reached from a loaded program; an empty stack
+        // here means malformed user-supplied bytecode, which must still surface
+        // as a typed error rather than a panic — including in debug builds.
         self.comprehension_stack.pop().map_or_else(
             || {
                 Err(VmError::InvalidIteration {