[grade=B urn:ump:wokpmecjfauc7jvnxyyyu4fq3zkkvwge455a5mxlmi3pdzkfdw3a] feat(DC-171): DNS provider plug-in parity phase 1 — BIND 9 AXFR listing + capability honesty

RFC2136/BIND9 adapter gains listRecords via AXFR zone transfer (dig with
the same TSIG identity as nsupdate; temp 0600 keyfile, always cleaned up;
transfer-denial errors carry the exact allow-transfer fix). TSIG fields
validated (name charset, HMAC algorithm whitelist, base64 secret) before
keyfile interpolation.

Capability honesty at every surface: GET /dns/records 501s name the active
provider + its supported capabilities + zone-transfer pointer; MCP
dashcaddy_list_dns attaches actionable guidance for AI clients; setup
wizard shows per-provider capability hints at choice time (parity-pinned
by tests against the adapters).

Also fixed a latent rfc2136 bug: this._log instance property shadowed the
prototype logging method — every nsupdate call threw since the DC-170-era
refactor. CHANGELOG stale 'protocol-impossible' DC-171 entry superseded.

Judge: grade B, zero blockers (urn:ump:wokpmecjfauc7jvnxyyyu4fq3zkkvwge455a5mxlmi3pdzkfdw3a).
Polish applied post-grade: stale changelog entry corrected, TSIG field
validation + 4 extra failure-path tests (exact dig argv, ENOENT install
hint, non-denial failure passthrough, hostile-field rejection),
dns-records 501 test strengthened to pin provider name + capabilities.
Suite: backend 174 suites / 3754 tests, frontend 68/68.

status/index.html

@@ -506,9 +506,10 @@
             <select id="setup-dns-provider" class="form-input-lg" style="margin-bottom: 12px;">
               <option value="technitium">Technitium DNS (recommended)</option>
               <option value="cloudflare">Cloudflare DNS</option>
-              <option value="rfc2136">RFC 2136 (BIND / PowerDNS / other)</option>
+              <option value="rfc2136">RFC 2136 (BIND 9 / PowerDNS / other)</option>
               <option value="manual">Manual / External DNS</option>
             </select>
+            <div id="setup-dns-provider-hints" class="form-hint" style="margin-bottom: 12px; line-height: 1.5;"></div>
             <div style="display: grid; grid-template-columns: 1fr auto; gap: 8px;">
               <input type="text" id="setup-dns-ip" value="" placeholder="DNS server IP"
                      style="padding: 12px; background: var(--card-bg); color: var(--fg); border: 1px solid var(--border); border-radius: 6px; font-size: 1rem;" />