refactor: consolidate RVM instruction variants and clean up VM internals

[anakrish]

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 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.