Reject malformed 32-bit constants in AND/XOR and SHA256 compilation [LA_Suggestion3]

[rose2221]

• Add 32-bit width assertions before as u32 truncation in process_binop_opcode (3 sites) to reject constants exceeding 32 bits at compile time instead of silently truncating
• Add matching assertion in cow_to_digit in binops.rs for self-contained validation independent of callers
• Add 32-bit width assertion in sha256_compression.rs for hash value constants used in the optimization path
• Refactor all assertion sites to bind into_bigint() once and reuse, avoiding redundant Montgomery-to-integer conversions