Add 1.7.0 release notes

[drudge]

User-facing notes for 1.7.0, consolidated from 1.6.2-beta.1 and 1.7.0-beta.1 through beta.8 for people upgrading from 1.6.1.

• Leads with what users notice: block lists apply correctly, DNSSEC false-Bogus fixes (RFC 4035 §§3.1.3.2, 5.3.4, 5.4; RFC 5155 §§7.2.6, 8.8), ExternalDNS on Forwarder zones, startup/memory/Insights speed, stability hardening, console polish.
• Drops internal work (component library, store/server/dnsserver refactors, test fixtures, render/API helpers) and beta-only regressions (Delete Record / DNSSEC key buttons broken in beta.1, fixed in beta.5).
• Upgrading section merges 1.6.2-beta.1, beta.1, and beta.3 notes.
• Moves docs version references to 1.7.0 (docs tests require it). The docs screenshot line still says 1.6.1 since they weren't recaptured.

Date is set to 2026-10-07; adjust if the release slips.