Fixes applied to Rust, Julia, and R AVM ISA ports:
1. Division rounding: use floor division (matching Lean Int.ediv)
2. Clamp range: symmetric [-2147483647, 2147483647] for Q16_16,
[-32767, 32767] for Q0_16 (preserves negation involution)
3. V6 sign-decomposition comparison for ltQ16
4. Stack depth limit: maxStackDepth = 1024 with StackOverflow error
5. Added AVM-specific constants and helpers to each port
All three ports now match the Lean reference specification.
