feat: build bdk-ffi for wasm32-unknown-unknown

[bennyhodl]

Description

Make the crate compile for wasm32-unknown-unknown, so that a UniFFI binding generator such as uniffi-bindgen-react-native (wasm2 flavor) can produce browser and Node bindings from the same API the native bindings use. Native targets are unchanged.

How the wasm package gets built and published is still open. That discussion lives in this gist: https://gist.github.com/bennyhodl/e290b7de4ee8c746ee28f19695ab7bd2. Whatever is decided there, the cfg splits in this PR are required for any wasm build, because rusqlite, minreq, bdk_electrum and bdk_kyoto have no wasm32 support and cannot be replaced from a downstream package.

What changes:

• Cargo.toml: rusqlite, the blocking Esplora client, bdk_electrum and bdk_kyoto move to a cfg(not(target_arch = "wasm32")) table. A wasm32 table adds the async Esplora client, getrandom/js, UniFFI's wasm-unstable-single-threaded feature, uniffi-runtime-wasm, and wasm-bindgen/js-sys/futures-channel for the sleeper. resolver = "2" stops features from unifying across target tables.
• esplora.rs: on wasm32 EsploraClient wraps AsyncClient. Every method keeps its name, arguments and result but is async. The constructor returns a Result because building the reqwest client can fail. Retry backoff sleeps through the host's global setTimeout, so it works in browsers, workers and Node. The request/update helpers are shared by both implementations.
• store.rs: Persister::custom is available on every target. The SQLite constructors and the pre-v1 migration helper are native only.
• error.rs: EsploraError gains a Reqwest variant. It exists on every target so the generated API is identical, but native never produces it. Electrum, SQLite, migration and Kyoto conversions are native only.
• lib.rs: the electrum and kyoto modules are not compiled on wasm32. An extern crate uniffi_runtime_wasm line keeps the runtime exports from being dropped by the linker.
• CI: a new wasm job runs cargo check --locked and cargo clippy --locked for wasm32-unknown-unknown.

Notes to the reviewers

• wasm-unstable-single-threaded is not a threading model choice. It only removes the Send + Sync bounds UniFFI requires on native, where foreign threads call in. wasm32-unknown-unknown has no threads. The Persistence trait keeps its Send + Sync bounds and compiles as is.
• secp256k1-sys needs a C compiler with a wasm32 backend. Ubuntu's clang has one, so CI sets CC_wasm32_unknown_unknown=clang. Apple's clang does not, so on macOS point that variable at an LLVM clang (for example Homebrew's llvm).
• Feature isolation was verified with cargo tree: native has no reqwest or wasm-bindgen, and wasm32 has no rusqlite or minreq.
• The proxy argument on the wasm32 EsploraClient constructor is kept for API parity and does nothing on that target.
• No CHANGELOG.md edit in this PR, following the release flow where entries come from the changelog: label.

Documentation

• Other:
  • esplora-client AsyncClient: https://docs.rs/esplora-client/0.12.3/esplora_client/async/struct.AsyncClient.html
  • esplora-client Sleeper trait: https://docs.rs/esplora-client/0.12.3/esplora_client/async/trait.Sleeper.html
  • bdk_esplora EsploraAsyncExt: https://docs.rs/bdk_esplora/0.22.2/bdk_esplora/trait.EsploraAsyncExt.html
  • uniffi-runtime-wasm: https://docs.rs/uniffi-runtime-wasm/0.31.0-5/uniffi_runtime_wasm/
  • uniffi-bindgen-react-native wasm2 reference: https://github.com/jhugman/uniffi-bindgen-react-native/tree/main/docs/src/reference/wasm2
  • Build and publish proposal: https://gist.github.com/bennyhodl/e290b7de4ee8c746ee28f19695ab7bd2

Changelog

changelog: added

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label (needs a maintainer: changelog: added)
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature. The CI job checks and lints the wasm32 target. A runtime test needs a JavaScript host and lives with the binding generator, not in this crate. The native test suite is unchanged.
• I've added docs for the new feature

[Ugarba202]

This is a massive step forward for the JS/web ecosystem!
I pulled the branch locally and ran through some checks:

• Dependency Isolation: Verified the cfg feature gating in Cargo.toml. The resolver = "2" addition does a great job at isolating the features.
• Wasm Compilation: Ran cargo clippy --target wasm32-unknown-unknown and it compiled flawlessly without any feature leaks.
• Native Integrity: Ran the standard cargo test suite on the Native targets just to be extra safe, and everything passed perfectly. No regressions for existing mobile/native users.
  The async translation for the Esplora client and the custom setTimeout sleeper wrapper for the retry backoffs look really solid. Exposing the Reqwest error variant globally to keep the UniFFI API shape identical across platforms is also a smart tradeoff.
  Everything looks great on my end. Thanks for putting this together!