v3.28.0 review: the batch failure classification

- A non-ValueError exception from the install run (a card removed
  mid-chain, a transport error) was recorded as a 'skipped' attempt with
  the error text - but the chain had started and died, which is a
  failure.  Only a ValueError (a request/eligibility error: the preset's
  ISD keyset, the plan) is a skip now; anything else is a 'failed' record
  carrying its error.  The PWA's failed row falls back to that error when
  there is no failing step / status word to show.
- The panel's Clear button is disabled while a batch is active (the
  server refuses the clear with 409 anyway) - no pointless error alert.
- api.md and the repo AGENTS document the two outcomes.  Tests: a
  transport error mid-chain is a failure (and the card stays seen), the
  plan's input validation (the block size, a missing cap_hex), the failed
  row without a step, the shared ram_format field, and the Clear state.
  915 frontend + 921 Python; sw cache simple-v438 (unchanged).

frontend/index.html

@@ -10213,6 +10213,9 @@ function ramBatchRender() {
 		stopBtn.textContent = phase === 'stopping' ? t('Stopping...') : t('Stop');
 		stopBtn.disabled = phase === 'stopping';
 	}
+	// the server refuses a clear while the run is active
+	const clearBtn = document.getElementById('ram-batch-clear');
+	if (clearBtn) clearBtn.disabled = active;
 	const stateEl = document.getElementById('ram-batch-state');
 	if (stateEl) {
 		if (!active) {
@@ -10261,7 +10264,7 @@ function ramBatchAttemptsHtml(attempts) {
 		const tone = a.result === 'installed' ? 'text-emerald-600 dark:text-emerald-400'
 			: (a.result === 'failed' ? 'text-red-600 dark:text-red-400' : 'text-amber-600 dark:text-amber-400');
 		const detail = a.result === 'failed'
-			? [a.failed_step, a.por_sw || a.sw].filter(Boolean).join(' \u2014 ')
+			? [a.failed_step, a.por_sw || a.sw || a.error].filter(Boolean).join(' \u2014 ')
 			: (a.reason || '');
 		html += '<tr class="border-b border-gray-100 dark:border-slate-800">'
 			+ '<td class="py-0.5 pr-2 text-gray-400 dark:text-slate-500 whitespace-nowrap">' + esc(when) + '</td>'