feat(external-signer): add Visibility arg to file_prepare_upload

[Nic-dorman]

Summary

Adds an external-signer pathway for public file uploads. Calling Client::file_prepare_upload_with_visibility(path, Visibility::Public) bundles the serialized DataMap into the existing payment batch (wave-batch or merkle) as one additional chunk. After finalize_upload / finalize_upload_merkle, FileUploadResult.data_map_address carries the chunk address of the published DataMap — a single network address the uploader can share so anyone can retrieve the file.

This unblocks public uploads in the Autonomi desktop GUI (ant-gui). The GUI currently has its Public upload option disabled in the UI (WithAutonomi/ant-ui#12, merged) because no external-signer pathway exists for publishing the data map. Once this lands and ant-gui bumps the git dep, a follow-up PR on that repo will thread the visibility flag through the Tauri commands and re-enable the Public button.

Why bundling, not a separate call

Publishing the data map means storing one extra content-addressed chunk. The two obvious alternatives both hit dead ends on the external-signer path:

• client.data_map_store(&data_map) after finalize_upload — what ant-cli does. Internally goes through pay_for_storage, which hard-requires self.require_wallet()? (ant-core/src/data/client/payment.rs:46). ant-gui's Rust client has no wallet — all payments go via WalletConnect / direct-wallet on the frontend.
• Hand-roll chunk storage externally. The internal chunk-storage helpers (store_paid_chunks, chunk_put_to_close_group, merkle_upload_chunks) are all pub(crate), so external callers cannot reproduce the flow.

Bundling the data map chunk into the existing PaymentIntent (wave-batch) or merkle batch reuses the one-signature flow that already works for file chunks. The external signer pays for the data chunks and the data map chunk in a single on-chain transaction, and finalize_upload[_merkle] stores them all in the same wave.

Changes

• Visibility enum (Private default, Public) in data::client::file, re-exported from data::.
• Client::file_prepare_upload_with_visibility(path, visibility). The existing file_prepare_upload(path) delegates with Visibility::Private — signature and behaviour are preserved for existing callers.
• PreparedUpload.data_map_address: Option<[u8; 32]> carries the address between prepare and finalize.
• FileUploadResult.data_map_address: Option<[u8; 32]> is Some(addr) for public uploads. Pair it with data_map_fetch(&addr) + file_download(&data_map, dest) to retrieve the file.
• Both finalize_upload and finalize_upload_merkle propagate data_map_address; no extra network call needed, since the data map chunk is stored alongside the rest of the batch.
• The internal-wallet path (file_upload_with_mode) is unchanged — ant-cli continues to use file_upload + data_map_store for its public flow.

Test plan

• cargo fmt -p ant-core clean
• cargo check --workspace --all-features passes
• cargo clippy --tests -p ant-core --all-features -- -D warnings clean
• New e2e test test_file_prepare_upload_visibility in ant-core/tests/e2e_file.rs verifies:
  • Private prepare leaves data_map_address unset
  • Public prepare records the address, and the recorded address equals compute_address(rmp_serde::to_vec(&data_map))
  • For wave-batch, the Public path adds exactly one extra prepared chunk (the data map) to the batch, at the recorded address
• End-to-end verification on ant-gui lands in a follow-up PR (wiring visibility through Tauri commands and re-enabling the button)

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