feat(external-signer): emit UploadEvent progress through prepare/finalize

[Nic-dorman]

Summary

Adds optional Option<mpsc::Sender<UploadEvent>> parameters to the external-signer upload API so applications can drive a progress bar through both phases of the two-phase flow. Without a sender, behaviour is identical.

The wallet-flow methods (file_upload_with_progress, file_download_with_progress) already emit these events; this PR closes the symmetry gap so the external-signer flow emits the same events.

API additions

Each existing method gains a new sibling that accepts a sender (the existing method delegates with None):

existing	new
Client::file_prepare_upload	Client::file_prepare_upload_with_progress
Client::finalize_upload	Client::finalize_upload_with_progress
Client::finalize_upload_merkle	Client::finalize_upload_merkle_with_progress

Events emitted

event	when
UploadEvent::Encrypting { chunks_done }	once per 10 spilled chunks during encryption
UploadEvent::Encrypted { total_chunks }	once after encryption + the optional public DataMap chunk are accounted for — first point where the total is known
UploadEvent::ChunkQuoted { quoted, total }	once per chunk during the wave-batch quote-collection phase. Replaces the previous bulk .collect() with a streaming loop so events fire as each completes.
UploadEvent::ChunkStored { stored, total }	once per chunk during finalize. Wave-batch reuses the existing store_paid_chunks_with_events helper; merkle gets a new progress parameter on merkle_upload_chunks that emits the same event from its store loop.

Other touches

• Removed the dead store_paid_chunks no-events alias; finalize_upload_with_progress calls store_paid_chunks_with_events(.., None, 0, total) directly.
• merkle_upload_chunks gains a progress: Option<&mpsc::Sender<UploadEvent>> parameter. The other in-tree caller (Client::data_upload) passes None and is unaffected.

Test plan

• cargo check --workspace
• cargo clippy -p ant-core --all-targets -- -D warnings
• cargo fmt --check
• cargo test -p ant-core --lib — 221 pass; 1 unrelated pre-existing failure (adaptive::tests::save_snapshot_to_unwritable_dir_does_not_panic, Windows fs-permission flake — fails identically on main / rc-2026.4.3).
• End-to-end against a Sepolia-EVM devnet via a downstream consumer (ant-gui) wiring the new senders into its Tauri commands. Multiple 9-chunk uploads — every ChunkQuoted event reached the consumer with correct cumulative counts (0/9 → 9/9), and every ChunkStored event reached it during finalize (1/9 → 9/9). Confirmed via debug bridge that the receiver applies each event to the row's state.

Companion PR

Downstream consumer wiring is on a parallel branch (ant-gui), gated on this PR landing + an ant-core release tag.

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