Define and implement Whale lowering for Wave integers wider than 128 bits #884

Motivation

At fd07427f4a976de05e38eff3ee1760db97fbb761 on Linux amd64 with Rust 1.89 and locked Whale 020d2a4, lexing, parsing and TypedProgram lowering succeed, but calling whale::emit_ir on this source returns Whale backend does not yet support type Uint(256):

fun identity(x: u256) -> u256 { return x; }

Scope

Decide a Wave-to-Whale representation for existing 256–1024-bit integers, then implement operations/conversions without narrowing to 128 bits. Inspect upstream types and coordinate required capabilities; use Utils and compiler-owned freestanding arithmetic where appropriate. Full-value constant shift checks in #860 are not wider runtime support.

Difficulty: very hard.

Completion criteria

• Record the representation and backend capability contract.
• Preserve wrapping, conversions, comparisons and checked shift semantics.
• Provide boundary tests and explicit diagnostics for remaining operations.

Please mention @LunaStev to discuss the design before starting implementation.