fix: upload SNP chunks to initial witness group

[dirvine]

Summary

• Keep SNP quote collection on the quorum-witnessed candidate set.
• Change SNP PUT targets to the initial witnessed close group instead of the witnessed/quoted set.
• Preserve median-issuer voter ordering so the initial PUT wave prefers witnesses that voted for the paid median issuer.

Why

The receiver-side gate in ant-node is intentionally strict: a node should only accept storage for chunks it considers itself responsible for. The client was collecting quotes from the witnessed consensus set and then uploading to that quoted set. Under NAT/reachability divergence, that can select peers outside the initial responsible close group and hit receiver-gate rejections.

This PR keeps the quote group unchanged, as requested, but aligns the actual PUT targets with the initial/witness close group.

Verification

• cargo test -p ant-core --lib data::client::quote -- --nocapture
• cargo check -p ant-core
• cargo fmt --all -- --check
• git diff --check

[dirvine]

Updated per Mick's follow-up:

• Witnessed candidates are now sorted by XOR closeness as the primary sort before quote collection.
• SNP quote collection now tries the closest CLOSE_GROUP_SIZE witnessed peers first and only falls back to further witnessed peers if a closer peer fails to provide a usable quote.
• Merkle/fault-tolerant quote preflight keeps the previous concurrent over-query behaviour.

Re-verified locally:

• cargo test -p ant-core --lib data::client::quote -- --nocapture — 23 passed
• cargo check -p ant-core
• cargo fmt --all -- --check
• git diff --check