feat: add onchain event listener

[coreyphillips]

Adds an on-chain transaction watcher for extended public keys. A caller starts a watcher for an xpub and receives typed WatcherEvents via an EventListener callback.

Description

• New EventListener trait (#[uniffi::export(with_foreign)]) for receiving typed callbacks, implementable from Swift/Kotlin/Python.
• New FFI functions: onchain_start_watcher, onchain_stop_watcher, onchain_stop_all_watchers.
• New WatcherParams record: watcher_id, extended_key, electrum_url, optional network / account_type (auto-detected from the key prefix when omitted), and optional gap_limit (defaults to 20).
• New WatcherEvent enum: TransactionsChanged { transactions, balance, tx_count, block_height, account_type }, Error { message }, Disconnected { message }, Reconnected.
• Implementation (src/modules/onchain/listener.rs):
  • Uses Electrum blockchain.scripthash.subscribe for detection; a short-timeout ping() drains pushed notifications off the socket without a fixed poll interval.
  • Runs the wallet + Electrum client (both !Send) on one dedicated std thread per watcher rather than occupying a Tokio blocking-pool slot for the watcher's lifetime.
  • Init ordering is sync → subscribe → sync to close the sync/subscribe race window.
  • Self-healing connection: lost connections trigger a Disconnected event and an exponential-backoff reconnect (capped at 60s) that emits Reconnected; a failed resync is retried so a popped notification is never dropped.
  • Per-watcher generation guard so a tearing-down watcher can't evict a newer watcher reusing the same watcher_id.
  • Gap limit is extended automatically as new addresses are used.
• New WatcherError variant on AccountInfoError.
• Regenerated Android/iOS/Python bindings and bumped version to 0.1.65.

QA Notes

• cargo check and cargo test pass.
• Start a watcher against a known xpub on an Electrum server and confirm:
  • An initial TransactionsChanged event arrives with the current balance/history.
  • Sending a new tx to a derived address fires a TransactionsChanged update within seconds.
  • Killing/restoring the Electrum connection produces Disconnected then Reconnected events, and state resyncs afterward.
  • onchain_stop_watcher / onchain_stop_all_watchers tear the watcher(s) down cleanly.
  • Starting a second watcher with an already-active watcher_id returns a WatcherError.

[coreyphillips]

• Release created as 0.1.65 here.
• Android PR for testing is available here.

[chatgpt-codex-connector]

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[ben-kaufman]

Found two watcher issues in the review.

[ben-kaufman]

Found one small lifecycle edge in the second pass.

[ben-kaufman]

One small perf note from the BDK/electrum-client pass.

[ben-kaufman]

One design question about keeping the watcher descriptor-first.

[ben-kaufman]

One small formatting note

[ben-kaufman]

One retry pacing note from the follow-up review.

[ben-kaufman]

One startup semantics question from the architecture pass.

[ben-kaufman]

One tip-height edge case from the architecture pass.

[ben-kaufman]

One small subscription nit.

[ben-kaufman]

LGTM, would be good to update to more descriptor focus architecture as a follow up.

[coreyphillips]

I'll bump the version and rebuild.

• New v0.1.66 release here.