refactor: consolidate RVM instruction variants and clean up VM internals (#651)

Merge the three separate Assert* instructions (AssertNot, AssertCondition,
AssertNotUndefined) into a single `Guard { register, mode }` instruction
with a GuardMode enum. This cuts duplicated match arms across display,
listing, parser, dispatch, and all compiler emit sites.

Drop the unnecessary `#[repr(C)]` from the Instruction enum. It was never
exposed across FFI, so the C-compatible 4-byte discriminant was pure waste.
Without it Rust picks a 1-byte discriminant, shrinking every instruction
from 8 bytes to 6. A new `instruction_size` unit test locks this at 6.

While touching these files, also clean up several long-standing issues:

- Deduplicate the iteration-state setup in loops.rs by extracting a shared
  resolve_iteration_state() helper -- the stack-based and stackless paths
  had near-identical 40-line blocks.
- Collapse the ExitWithSuccess / ExitWithFailure match arms into one.
- In rules.rs, stop cloning Arc<Program> just to borrow a RuleInfo -- clone
  the small RuleInfo struct directly and extract a get_rule_info() helper.
- Move the memory check into dispatch (runs per instruction) and remove the
  now-dead enforce_memory_check() entry-point calls.
- Apply map_or_else style throughout listing.rs for consistency.

src/rvm/vm/comprehension.rs

@@ -435,6 +435,8 @@ impl RegoVM {
             }
         }
 
+        self.set_register(result_reg_idx, current_result)?;
+
         let (iteration_state_snapshot, body_start, comprehension_end) = {
             let frame = self.execution_stack.get_mut(comprehension_index).ok_or(
                 VmError::InvalidIteration {
@@ -485,8 +487,6 @@ impl RegoVM {
             }
         };
 
-        self.set_register(result_reg_idx, current_result)?;
-
         if let Some(state) = iteration_state_snapshot.as_ref() {
             let has_next = self.setup_next_iteration(state, key_reg_idx, value_reg_idx)?;