Retire rain.lib.hash after rainlang.interface adopts EIP-712 signed context: inline the one hashWords line and drop the dependency

[thedavidmeister]

Blocked by rainlanguage/rainlang.interface#133.

Why

The only source file in the org that calls this library is rainlang.interface's LibContext.sol, which uses hashWords(context) for the signed-context digest and imports HASH_NIL as a fold seed; its two fold functions already inline the composition pattern by hand. Seven other repos declare the soldeer dependency only because non-recursive soldeer makes them restate rainlang.interface's dependencies; none imports it. (Verified 2026-09-03 by GitHub code search plus direct fetch of the candidate files; historic OrderBook deployments embed the library's bytecode and are unaffected.)

Once #133 moves signed context to EIP-712 hashStruct, the library's contribution to the stack is one assembly line: keccak256(add(ptr, 0x20), mul(mload(ptr), 0x20)), which is EIP-712's own encoding of a bytes32[] member. HASH_NIL is keccak256("") and folds at compile time; combineHashes is two mstores and a keccak. A versioned package, its audit surface and its release lifecycle for one line is the wrong side of the ledger.

Done when

• rainlang.interface inlines that line in LibContext (or its V2 successor) and drops rain-lib-hash from foundry.toml / soldeer.lock; the seven transitive listings disappear with it as those repos re-lock.
• This repo's README pattern section is either archived as-is or its encodeData-relevant parts move next to the implementation in rainlang.interface.
• This repo is archived on GitHub with a README banner pointing at rainlang.interface.

Refs #7 (moved to rainlang.interface #133), #22.

[thedavidmeister]

Misplaced: dropping the dependency is rainlang.interface's work and is already a follow-on on rainlanguage/rainlang.interface#133. Nothing for this repo to do until then.

[thedavidmeister]

Reopening: this is now required by a specification rather than being a tidy-up that follows #133 whenever it happens.

S01-Issuer/st0x.attest SPEC.md item 19: "Signed context moves off the hash library and onto EIP-712 signed data, and the hash library is removed." The removal is named in the spec as part of the same change, not as a follow-on that may or may not get done.

The shape is unchanged from what this issue already describes — once rainlanguage/rainlang.interface #133 lands, the library's remaining contribution to that stack is the one hashWords line, which gets inlined so the dependency can go.

Spec: https://github.com/S01-Issuer/st0x.attest/blob/main/SPEC.md