receipts [decide]: default anchor tier + checkpoint cadence as a measured SLA (D3)

[bordumb]

From docs/plans/tools/auths-receipts.md (open questions):

• Which anchor tier is the default when more than one is available (treasury / witness / onchain / first-seen)?
• The treasury/witness checkpoint cadence bounds escrow settlement latency: every objection window wᵢ must exceed the cadence, so the cadence is a measured product SLA — benchmark it and publish the minimum escrow window it implies.
• Pricing of a per-settlement anchor vs a batched one.

Landed so far on dev-receipts: treasury + first-seen tiers wired in auths-evidence::anchor (witness/onchain fail closed as unsupported — see the deferred-scope issue); escrow_open enforces wᵢ > cadence with a configurable AUTHS_ESCROW_ANCHOR_CADENCE_SECS (default 5s, matching the treasury coordinator's default checkpoint-secs).