feat: stream transactions and progress during sync

[ANAMASGARD]

Closes #2202

Description

Adds incremental sync progress and transaction streaming to Electrum and Esplora backends.

Wallets can register .on_event() on SyncRequest / FullScanRequest to receive:

• PartialUpdate(TxUpdate) — newly discovered txs/metadata at fetch boundaries
• AnchorsResolved(TxUpdate) — anchor-only delta after Electrum resolves confirmation proofs
• ItemStarted / SpkStarted — progress events (.inspect() is sugar over these)

TxUpdate::drain_since() destructively moves data out of the working buffer at each emission boundary. When events consume everything, response.tx_update is naturally empty — applying it at the end is a safe no-op (no double-apply footgun).

Consumer contract:

let request = SyncRequest::builder()
    .spks(spks)
    .on_event(|event| match event {
        SyncRequestEvent::PartialUpdate(delta)
        | SyncRequestEvent::AnchorsResolved(delta) => graph.apply_update(delta),
        _ => { /* progress UI */ }
    })
    .build();

let response = client.sync(request, batch_size, true)?;
graph.apply_update(response.tx_update); // empty when events drained all data

Emission points:

• Electrum: after each SPK batch; once after txid/outpoint passes; AnchorsResolved after batch_fetch_anchors
• Esplora: after each parallel batch (blocking + async)

Existing sync/full_scan signatures are unchanged (additive API).

Notes to the reviewers

• .inspect() now routes through .on_event() internally (single reporter path); progress-only users are unaffected unless they also call .on_event().
• emit_partial_updates defaults to false — draining only happens when .on_event() is used.
• Electrum confirmed txs in PartialUpdate intentionally lack anchors until AnchorsResolved; see sync_defers_anchors_until_resolved test.
• Esplora async uses EmitPartial trait on the concrete request (not dyn across .await) to satisfy Send.

Changelog notice

• Added SyncRequestEvent, FullScanRequestEvent, FullScanProgress, and TxUpdateCursor types.
• Added SyncRequestBuilder::on_event() and FullScanRequestBuilder::on_event().
• Added TxUpdate::drain_since() for incremental delta extraction.
• Electrum and Esplora now emit PartialUpdate events during sync/full_scan when .on_event() is registered.

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

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## What each test proves for #2202

| Test | Proves |
|---|---|
| `drain_since_*` | Delta extraction is correct; partials reconstruct full update |
| `final_response_empty_after_full_drain` | No double-apply when streaming |
| `sync_on_event_fires_item_started` | `.on_event()` builder works |
| `sync_streams_partial_updates` (electrum + esplora) | `PartialUpdate` fires **before** sync returns; final `tx_update` empty |
| `sync_defers_anchors_until_resolved` | Electrum anchor semantics correct |