Add ADR-0005 client-side earned reward eligibility gate

[grumbach]

Summary

Client-side of ADR-0005 "earned reward eligibility". Collects the signed audit reports that quote responders now ship and gates quote collection on them, so a client prefers to pay nodes that have earned their place (a clean audited week at the quoted size).

This is earned future eligibility, not slashing: it only affects who a client chooses to pay next, never settled payments.

What it does

• Eligibility predicate: majority-of-opinions vouch bar (floor 3), with a two-tier fallback (dues-at-any-size, then today's ungated rules) so the network never stalls under fast growth or churn.
• Report verification: each report's signature, request nonce, and size caps are checked before its rows are trusted. Payee selection and payment verification are untouched — the gate only narrows the candidate list at collection.
• Observe-only by default so shadow eligibility can be calibrated before any hard gate is enabled.

See docs/adr/ADR-0003-client-earned-reward-eligibility-gate.md (client predicate) and the canonical ant-node ADR-0005 for the full design.

Coordinated cutover

Part of the ADR-0005 wire cutover. Pins ant-protocol and ant-node to the immutable PR revs for a reproducible build; the [patch.crates-io] block is marked STRIP BEFORE MERGE and is replaced by the published ant-protocol 3.0.0 at release.

Depends on:

• Add ADR-0005 audit-report wire types for earned reward eligibility ant-protocol#19 (audit-report wire types)
• Add ADR-0005 node-side audit tally for earned reward eligibility ant-node#169 (node-side audit tally)

[grumbach]

Parking ADR-0005 (earned reward eligibility) for now: deprioritized while we focus on the ADR-0004 production fixes and capacity recovery. Closing to keep the review queue tidy, will reopen when we revisit. Branch stays intact.