Ad engine v1: banner/text/login campaigns against on-chain credits

Spec 8b types 1-3. Spend accrues per campaign in batches; burns queue for
the engine signer (admin runs consume() on-chain, /api/admin/burns). Rates
are volume config (adrates.json), rehearsal placeholders until Marty sets
the real card. Public slots serve on the ledger page; campaign manager in
the members area.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/ledger.html

@@ -15,9 +15,11 @@
     <p><span class="badge" id="liveBadge">connecting…</span>
        <span class="small muted" id="statLine"></span></p>
   </section>
+  <div class="card" id="adSlotBanner" hidden></div>
   <div class="card" style="padding:0">
     <div class="feed" id="feed"><div class="row muted">Loading recent history…</div></div>
   </div>
+  <div class="card small" id="adSlotText" hidden></div>
   <footer>
     <div>InstantAdPay · <a href="/">how it works</a> · <a id="contractLink" href="#" target="_blank" rel="noopener">contract source ↗</a></div>
   </footer>