External-signer merkle finalize reports success on partial chunk storage

[Nic-dorman]

Summary

The external-signer merkle finalize path returns Ok(FileUploadResult) even when chunks remain short of quorum after all retries — the failure is only visible in the chunks_failed field, which callers can (and do) ignore. The external-signer wave-batch path returns Err(Error::PartialUpload) in the same situation, so the two paths have opposite contracts for the same failure.

Verified identical in the published ant-core 0.5.0 and 0.5.1.

Where

• Client::merkle_upload_chunks (ant-core/src/data/client/merkle.rs) only raises Err for fatal non-quorum errors (missing proof, chunk/address count mismatch). Chunks still short of quorum after MERKLE_STORE_MAX_ATTEMPTS (the InsufficientPeers case) are returned as counts in MerkleStoreOutcome::failed / failed_addresses.
• Client::finalize_upload_merkle_with_progress (ant-core/src/data/client/file.rs) maps that straight into Ok(FileUploadResult { chunks_stored, chunks_failed, .. }) with chunks_failed > 0.
• Contrast: Client::finalize_upload_with_progress (wave-batch) returns Err(Error::PartialUpload { stored, failed, .. }) if any chunk failed.

The doc comment on MerkleStoreOutcome::failed_addresses acknowledges the gap ("Used by the CLI path to build PartialUpload; the external-signer path only reads the counts"), and the comment above the outcome.fatal re-raise claims the external-signer path is "all-or-nothing", which the code doesn't deliver for quorum shortfalls.

Impact

Merkle is auto-selected for uploads of ≥64 chunks, so this affects exactly the large paid uploads where partial storage is most likely and most expensive:

• The desktop app calls finalize_upload_merkle_with_progress, gets Ok, marks the upload complete, persists the DataMap, and shows a shareable address — for a file that cannot be fully downloaded. The user has paid on-chain.
• The mobile FFI wrapper converts FileUploadResult to its result type and drops chunks_failed entirely, so iOS/Android external-signer integrations report success on partial storage the same way.
• The only trace is the log line External-signer merkle upload finalized: N chunks stored, M failed.

Proposed fix

When outcome.failed > 0 in the external-signer merkle finalize, return Err(Error::PartialUpload) mirroring the wave path:

• stored = already_stored_addresses + outcome.stored_addresses, stored_count = outcome.stored
• failed = outcome.failed_addresses, failed_count = outcome.failed
• total_chunks as prepared; spend unknown to the library on this path (external signer pays out-of-band)
• update the # Errors docs and the two comments above so the contract is stated

The plumbing already exists — failed_addresses carries (address, last error) pairs.

Relationship to #140

This makes the failure visible; #140 is about making it recoverable (post-payment finalize is not retryable, so a PartialUpload after payment currently strands the payment). Fixing this one first is still strictly better than silent success: merkle proofs are reusable, so #140's resolution can later turn the surfaced PartialUpload into a free re-store instead of a re-payment. The wave path already has exactly this visibility/retryability split today.