fix(client): fetch witnessed quotes concurrently

[mickvandijke]

Summary

• keep up to CLOSE_GROUP_SIZE witnessed quote requests in flight concurrently
• launch fallback witnessed quote requests only when an in-flight peer fails to produce a usable quote
• preserve exact CLOSE_GROUP_SIZE successful quote selection and existing Merkle preflight behavior

SemVer

• patch: fixes SNP quote collection latency/behavior without changing public API

Tests

• cargo fmt --all -- --check
• cargo test -p ant-core quote --lib
• cargo clippy --all-targets --all-features -- -D warnings

[dirvine]

Hermes review

Verdict: looks good / ready from my review.

I reviewed the single-file change in ant-core/src/data/client/quote.rs with the SNP/witnessed quote invariants in mind.

Checks verified:

• SNP/witnessed collection now keeps up to CLOSE_GROUP_SIZE quote requests in flight concurrently.
• Fallback witnessed quote requests are only started after an in-flight peer fails to produce a usable quote (AlreadyStored, bad binding/protocol/network/timeout), not after a successful quote.
• Final proof construction still returns/selects exactly CLOSE_GROUP_SIZE quotes and preserves the median-voter support requirement.
• The 2x/fault-tolerant quote path remains isolated to Merkle preflight/probing and does not feed SNP payment proofs.
• Completion order does not define final quote order; existing distance/median-voter selection still applies after collection.
• Merkle preflight keeps the previous full concurrent over-query behaviour.

Validation run locally on head d1dca7c314fed59cf22ebe1444da4b3ca5b3f426:

• cargo fmt --all -- --check ✅
• cargo test -p ant-core quote --lib ✅ — 32 passed
• cargo clippy -p ant-core --all-targets --all-features -- -D warnings ✅
• git diff --check origin/rc-2026.6.2...HEAD ✅

GitHub CI is also green, including unit, E2E, Merkle E2E, clippy, format, docs, build, and security audit.

Non-blocking note: deterministic unit coverage for the new refill scheduler would be nice longer-term, but I don’t see a concrete correctness risk here and the current change is small/bounded.