Make the u32 value domain a compile-time fact for MemwOperation

[MauroToscano]

Third follow-up from the #786 review, stacked on #791. Types-only change; produced traces are byte-identical.

#786 shrank MemwOperation.value/old to [u32; 8] but kept [u64; 8] constructor signatures, narrowing with debug_assert! + as u32 — release builds would silently truncate an out-of-domain value (a bus-imbalance debugging nightmare if a future caller ever passes a full 64-bit word), and the M1/M3/M5 register fast path cast as u32 at its call sites with no assert at all, in any build profile.

This moves the domain to the type system instead of guarding it at runtime:

• MemwOperation::new and with_old take [u32; 8] directly; the narrowing maps and their debug_asserts are deleted. (old_timestamp stays [u64; 8] — timestamps can reach u64::MAX.)
• pack_register_value returns u32 limbs, so the fast path's bare casts disappear rather than gaining asserts.
• MemoryState::read_bytes returns [u32; 8] values (they are u8 memory bytes), and the byte-domain call sites (load, store, commit, keccak, ECSM) build [u32; 8] at the source.
• LoadOperation keeps its existing u64 interface via a widening map(u64::from) at the boundary.

Out-of-domain values are now unrepresentable in every build profile — the "error cleanly" version of the guard, at zero runtime cost.

Testing

• make lint passes; cargo clippy --all-targets no warnings; cargo fmt --check clean.
• Full cargo test -p lambda-vm-prover --lib: 503 passed — identical to the pre-change baseline (the 5 failures are the known missing rust-guest ELF fixtures, failing at file-read before any prover code runs; CI builds them and runs the full suite).

[MauroToscano]

Rebased on the updated branch. Note on 3b6840e (which upgraded the narrowing guards to always-on assert!): this PR resolves the same concern one level deeper — with [u32; 8] at the constructor boundary the assert becomes tautological and is deleted. Out-of-domain values are unrepresentable in every build profile, with no release-mode panic path in the trace-gen hot loop.