cow-venue: use alloy primitives instead of byte-array aliases

[mfw78]

What

Replaces cow-venue's pub type Address = [u8; 20] / pub type U256 = [u8; 32] aliases with alloy_primitives::Address/U256 throughout the order body (SellToken/BuyToken, the OrderBody fields, the builder constructor) and every dependent, dropping the to_be_bytes/from_be_bytes/into_array hops in assembly.rs and the adapter that existed only to cross the old alias boundary. The value-flow header and quotation projections now go through a new AssetAmount::erc20(Address, U256) constructor on videre-sdk, which owns the minimal big-endian uint encoding next to the wire type it serves, so the adapter no longer hand-rolls to_be_bytes_trimmed_vec and its call sites read in plain alloy types (value_flow becomes a real module re-exporting the bindings, so its public path is unchanged). Borsh support for the alloy types comes from alloy-primitives' own borsh feature (the FixedBytes/Address impls live in-crate and its feature passes through to ruint/borsh for U256), the same pattern composable-cow already uses. The U256 borsh wire form moves from 32-byte big-endian to ruint's little-endian, so the published codec vectors and header goldens are regenerated; the conformance suite gains regeneration-parity tests and an ignored regenerate_published_fixtures writer matching the videre-test house pattern. The issue's two hygiene points are settled as well: alloy-primitives is regated from the assembly slice into the body slice (kept optional only because it gates the empty --no-default-features build), and the no_std cfg_attr plus the extern crate alloc plumbing are dropped since every target compiles the crate with full std over wasm32-wasip2.

Why

The aliases shadowed alloy primitive names with raw byte arrays, forcing byte-hop projections at every chain edge and keeping the body types unnaturally weakly typed. Nothing is released, so the wire may move freely with no body-version bump.

Closes #546

Testing

nix develop --command just ci passed (fmt, clippy -D warnings, doc, the module wasm builds, cargo test --workspace --all-features). nix develop --command just check-venue-agnostic and nix develop --command just check-cow-orderbook-only both passed. The regenerated fixtures round-trip through the shipped codec and the adapter's own mainnet derivation.

AI Assistance

Implemented with Claude Code.