feat(wit): add nexum:value-flow types package v0.1.0

[mfw78]

What

Adds the nexum:value-flow WIT types package at 0.1.0, the shared,
dependency-free vocabulary for value in motion (settlement, asset,
asset-amount) that intent headers, simulation balance diffs, and analyser
verdicts will all use to describe a spend the same way.

• Adds wit/nexum-value-flow/types.wit defining interface types with:
  • settlement: evm-chain(u64) | offchain(string)
  • service-desc: kind, summary
  • offchain-desc: domain, summary
  • asset: native-token(settlement) | erc20/erc721/erc1155 as
    (chain id, address[, id]) tuples | service(service-desc) |
    offchain(offchain-desc)
  • asset-amount: asset plus a big-endian unsigned-byte amount
• The package carries no dependency (bare u64, no import of
  nexum:host/types), per the zero-dependency foundation mandate.
• Adds a #[cfg(test)] wasmtime bindgen smoke in
  crates/nexum-runtime/src/bindings.rs that imports the interface through a
  throwaway inline world and names every generated type, variant, and field
  by its plain Rust spelling, enforcing the identifier-hygiene gate at
  compile time.

Why

This is the platform's hardest-freezing contract: the vocabulary for where a
deal settles, what asset moves, and how much of it, shared by intent
headers, simulation balance diffs, and analyser verdict subjects so a spend
is written the same way everywhere. Identifier hygiene is a freeze gate:
every identifier is checked against WIT keywords (including in-flight
proposals) and against the reserved words of the nine binding-target
languages, so a two-word kebab id is preferred wherever a single word is a
keyword anywhere (native-token not native, offchain not external).
Getting this package right now, before anything binds to it, avoids
escaped identifiers surfacing later in generated bindings for the personas
the SDK exists to serve.

Testing

• cargo test --workspace (covers the new bindgen smoke test)
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings

Red-teamed with no blocker or major findings. The identifier-hygiene gate
was confirmed sensitive by injecting a Rust-keyword field name (kind to
type), which forced an r# escape and failed compilation; the file was
then restored to the exact committed state (git diff HEAD empty).

AI Assistance

Claude (Opus) used for the implementation.

Closes #226

[lgahdl]

Clean package and good hygiene gate. Three items below.

[lgahdl]

Two issues with native-token(settlement):

1. The BZZ example is wrong. BZZ is an ERC-20 on Ethereum/Gnosis Chain, not the native gas token of any chain. Correct examples: ETH, xDAI, MATIC, BNB.

2. native-token(offchain(...)) is a structurally valid but semantically empty combination. No real domain has an off-chain-settled native gas token; an analyser receiving this case has no oracle, no AMM pool, and no contract address to price against. The rationale for wrapping settlement is that each EVM chain has its own native token — that maps only to settlement::evm-chain. Either restrict native-token to hold a bare u64 (consistent with ERC cases), or add a comment explicitly forbidding the offchain arm and stating what consumers must do if they see it.

[lgahdl]

The ERC cases use anonymous positional tuples for a package described as the "hardest-freezing contract." The second position (address) is already opaque; at a frozen interface, adding a third field means a semver bump even for optional metadata. Named records would make positions self-documenting and leave room for a non-breaking extension:

record evm-token   { chain-id: u64, address: list<u8> }
record evm-nft     { chain-id: u64, address: list<u8>, token-id: list<u8> }

erc20(evm-token),
erc721(evm-nft),
erc1155(evm-nft),

This is easier pre-freeze than post.

[lgahdl]

The amount field is missing its doc comment — the encoding convention (big-endian, empty = zero, no negative) is documented on the record but not on the field itself. WIT tooling renders field docs separately.

Suggested change

	amount: list<u8>,
	/// Big-endian unsigned integer, most-significant byte first; empty encodes zero.
	amount: list<u8>,