fix(snp): lower witness quorum for partial transcripts

[mickvandijke]

Semver: patch

Summary:

• warn when SNP witnessed close-group transcripts are missing responder views
• lower the SNP witness quorum by one for each missing K-close responder view, clamped to at least one
• apply the adjusted quorum consistently to candidate selection, median-issuer support, and PUT peer ordering
• fetch SNP witnessed quotes concurrently with an exact 7-quote in-flight window: start with the closest 7 candidates, and launch the next closest fallback only when an in-flight peer fails or reports already-stored
• share per-peer quote request/result handling between SNP and Merkle preflight quote collection

Testing:

• cargo test -p ant-core witnessed_ -- --nocapture
• cargo test --locked -p ant-core witnessed_ -- --nocapture
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings

[dirvine]

Quick safety review from Hermes (requested from Slack):

Verdict: looks safe to merge once maintainers are happy. I did not find a blocking issue.

Checked:

• SNP/witnessed quote collection now keeps an exact CLOSE_GROUP_SIZE in-flight/success window: initial launch is the closest 7; fallback peers are only launched after a failed / unusable / already-stored outcome, not after a successful quote.
• Successful quote completion order does not define proof selection: quotes are still sorted/selected by XOR-distance and median-issuer voter support after collection.
• The 2 * CLOSE_GROUP_SIZE fault-tolerant path remains isolated to Merkle preflight / already-stored probing, not SNP SingleNodePayment proof construction.
• Adjusted quorum is applied consistently to candidate selection, paid-median support, and PUT peer ordering.
• Existing bad quote-binding filtering remains preserved via the shared request/result helper.

Local checks run on PR head 883f9043ff39a19a2b42b4eaf16f2e03ee07c431:

• cargo test -p ant-core witnessed_ --lib --no-fail-fast — pass (8 tests)
• cargo fmt --all -- --check — pass
• cargo test -p ant-core witnessed_quote_launch_budget_keeps_exact_quote_window --lib --no-fail-fast — pass
• cargo test -p ant-core quote_query_counts_keep_single_node_close_group_only --lib --no-fail-fast — pass

GitHub checks are green across build, unit, clippy, format, docs, security audit, E2E and Merkle E2E on the current run.

One note: I intentionally treated extra deterministic scheduler coverage as non-blocking here because the implementation is small/bounded and the key launch-window invariant has direct unit coverage.