[#1677] Add multi-server transaction submission

[czarcas7ic]

Closes: zodl-inc#1677

Depends on the auto/manual connection-mode branch.
Depends on the merged Swift SDK Broadcaster API work for the new SDK APIs.

Summary

• Transactions are now created locally and broadcast through the shared multi-server submission path
• First successful response wins, eliminating single-server failure as a blocker
• Remaining servers get a 5-second grace period for best-effort propagation
• 30-second global timeout as safety net
• Normal send, Keystone (PCZT) send, auto-shielding, Flexa, and swap-and-pay now use the same create-and-submit infrastructure
• Automatic mode broadcasts to all known servers; manual mode submits to the user's selected server only

Draft dependency note: Package.swift intentionally uses a local SDK path while this PR waits for a tagged Swift SDK release containing the Broadcaster APIs. The upstream merge commit is 348a9dfff2604f986590c64272ce12dd4a83c943, but pinning that commit directly is not buildable yet because the SDK source currently expects unpublished libzcashlc symbols. Before merge, this should be switched back to the tagged SDK release once available.

Behavior note: Per-transaction partial-submit status is no longer stored in SendConfirmation state. If a later transaction in a multi-transaction proposal fails after local creation, the send flow falls back to the existing pending/failure result presentation instead of surfacing per-transaction status strings.

Testing

• Wired the new APIs into zodl.
• Tested both regular and Keystone send paths with multi-server broadcast.
• Verified automatic mode (9 servers) and manual mode (1 server) on physical device.
• Added automated coverage for regular multi-transaction submission, all-server rejection, endpoint selection, direct submission helper behavior, first-success behavior, and the PCZT broadcaster/reset path.
• git diff --check
• swift package --package-path modules dump-package
• xcodebuild -list -project secant.xcodeproj
• Focused xcodebuild test is currently blocked before zodl code by the untagged SDK package artifact mismatch described above.

Video from testing:

https://screen.studio/share/1mgC0NYH

If you check the logs in the video, we can see a perfect example of why this feature is good, as one of the endpoints was faulty with an Orchard anchor error, but the other endpoints successfully submitted it.

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[czarcas7ic]

This will change once zcash/zcash-swift-wallet-sdk#1678 is merged and tagged.

[czarcas7ic]

Closing in favor of zodl-inc#1751