[RFP] LEZ-DK: The LEZ Development Kit

[fryorcraken]

One of five LEZ deliverables split out from #214 ("Review API reference and
requirements for Blockchain and LEZ integration"). No draft PR yet.

Scope

The LEZ Development Kit: one unified library per language carrying both the
LEZ wallet API and the LEZ node API, modelled on the Bitcoin Development Kit
(logos-co/rfp#200). It is what an application outside
Basecamp integrates LEZ with, whether the node runs in process or remotely.

The kit carries the FFI crates over the Rust wallet and node crates, plus the
transport client libraries, and ships as one artifact per language (Kotlin,
Swift and Go among them). Following BDK, the wallet and a client stay separate
objects that the integrator wires together rather than one reaching the other,
and separation is by object rather than by namespace.

Two scoping questions carried from RFP-027's design rationale:

• Whether the transport clients are gated behind build features. BDK links all
  of its chain backends unconditionally and pays for it in binary size; LEZ has
  one transport today and more anticipated ([RFP] Additional Interface Modules & External SDKs (gRPC, GraphQL, Mesh, Rust) #222), so deciding before there are
  several is cheaper than retrofitting.
• Whether the kit offers a sync convenience over the value-passing path. The
  lez_core module ships one, and BDK's absence of an equivalent is the gap
  its integrators wrap themselves.

⚠️ Not the Provider SDK

The LEZ-DK is not the same thing as the "Provider SDK" in LP-0021 / LP-0022 /
#194, despite both names containing "SDK". They sit on opposite sides of the
wallet boundary:

	The LEZ-DK	Provider SDK
What it is	What a wallet is built from	What a dApp calls to ask a wallet to sign
Keys	Holds them	Holds none
Consumed by	Whoever implements a wallet	dApp developers
Ecosystem analogue	BDK (Bitcoin)	EIP-1193 provider; Solana Wallet Adapter

Anyone scoping, reviewing, or awarding either one should confirm which side of
that boundary they are on first.

Source material

• appendix/wallet-libraries-ecosystem.md (in logos-co/rfp) — where wallet
  functionality lives per chain, key derivation standards, watch-only support,
  hardware and offline signing.

Dependencies

• LEZ Wallet API ([Internal RFP] LEZ Wallet API: key handling, derivation, proving, and signing #236) — the kit's wallet half sits over that FFI.
• LEZ Node API ([Internal RFP] LEZ Node API #235, RFP-027) — the kit's node half sits over that FFI, for
  an application embedding a node of its own.
• JSON-RPC Proxy Module & Client Library ([RFP] LEZ JSON-RPC Proxy Module & Client Library #237) — the kit carries that
  client for an application reaching a remote node.

Related

• Review API reference and requirements for Blockchain and LEZ integration #214 — parent tracking issue
• Sibling LEZ RFPs: LEZ Node API ([Internal RFP] LEZ Node API #235, RFP-027, rfp#200), LEZ Wallet API
  ([Internal RFP] LEZ Wallet API: key handling, derivation, proving, and signing #236), JSON-RPC Proxy & Client Library ([RFP] LEZ JSON-RPC Proxy Module & Client Library #237). The separate Indexer Client
  Library issue ([RFP] LEZ Indexer Client Library #239) was folded into [RFP] LEZ JSON-RPC Proxy Module & Client Library #237.
• [RFP] Additional Interface Modules & External SDKs (gRPC, GraphQL, Mesh, Rust) #222 — further transports, each adding a client the kit carries