v3.18.2: review fixes - sticky event-JSON error, store keep-on-load, write queue

The four findings from the review of the 3.15-3.18 batch:

- The event step's `_eventFieldsError` flag is cleared at the start of each
  collect: fixing malformed JSON used to leave the save refused until the
  editor was closed and reopened.  Regression test covers invalid -> fixed.
- The script store keeps an entry that no longer validates on load (a
  tightened engine rule or a hand edit): it is served, fails at run start with
  the validator's message and can be fixed in the editor, instead of being
  dropped from memory and erased by the next save.  Mutation paths still
  refuse it.  (The presets store already had this behaviour.)
- Writebacks are serialized per script (`_testWriteChains`): two quick saves
  of an unsaved draft no longer create two server-side scripts - the window
  could be minutes while a RAM operation holds the card lock.  Regression
  test asserts one create + one update.
- Deleting a script cancels its pending debounced save and marks it deleted,
  so a queued writeback never resurrects it; a create already in flight is
  undone with a delete.  Regression test covers the pending-save case.
- The event vectors moved to a shared fixture
  (frontend/tests/event_vectors.json) asserted by BOTH suites - a one-sided
  builder change now fails the other suite instead of drifting silently.

Tests: 816 frontend + 716 Python; sw cache simple-v383.

frontend/index.html

@@ -1877,7 +1877,7 @@
 // ===== Version =====
 // Single source of truth for the PWA version: shown in the header and used
 // by the server version check in pysimConnect().
-const SIMPLE_VERSION = '3.18.1';
+const SIMPLE_VERSION = '3.18.2';
 document.getElementById('app-version').textContent = 'v' + SIMPLE_VERSION;
 
 // ===== Tab switching =====
@@ -6878,6 +6878,10 @@ let _testCurrentIdx = -1;
 let _testRunState = null;
 let _testRunTimer = null;
 let _testSaveTimer = null;
+// One write chain per script object: saves of the same script run in order, so
+// two quick saves of an unsaved draft cannot both create it (the first assigns
+// the id before the second starts).
+let _testWriteChains = new WeakMap();
 let _testEditStep = null;
 let _testEditChecks = [];
 let _testLastPresetIdx = -1;
@@ -6937,11 +6941,23 @@ function testScriptsSave(immediate) {
 	const cur = testCurrent();
 	if (!cur) return;
 	if (_testSaveTimer) { clearTimeout(_testSaveTimer); _testSaveTimer = null; }
-	if (immediate) { testScriptsWriteback(cur); return; }
-	_testSaveTimer = setTimeout(() => { _testSaveTimer = null; testScriptsWriteback(cur); }, 600);
+	if (immediate) { testScriptsWritebackQueued(cur); return; }
+	_testSaveTimer = setTimeout(() => {
+		_testSaveTimer = null;
+		testScriptsWritebackQueued(cur);
+	}, 600);
+}
+
+// Serialize the writebacks of one script: the queued save reads `script.id`
+// only when it runs, so a create's id is adopted before the next update.
+function testScriptsWritebackQueued(script) {
+	const prev = _testWriteChains.get(script) || Promise.resolve();
+	const next = prev.then(() => testScriptsWriteback(script)).catch(() => {});
+	_testWriteChains.set(script, next);
 }
 
 async function testScriptsWriteback(script) {
+	if (script._deleted) return;   // queued before the delete: never resurrect
 	const problem = testScriptProblem(script);
 	if (problem) return;   // an incomplete draft stays local until it is valid
 	const payload = {name: script.name || '', steps: script.steps};
@@ -6952,7 +6968,13 @@ async function testScriptsWriteback(script) {
 		} else {
 			const resp = await pysimFetch('/api/test/scripts', payload);
 			if (resp.error) throw new Error(resp.error);
-			if (resp.script && resp.script.id) script.id = resp.script.id;
+			if (resp.script && resp.script.id) {
+				script.id = resp.script.id;
+				if (script._deleted) {
+					// deleted while the create was in flight: undo it
+					try { await pysimFetch('/api/test/scripts/delete', {id: script.id}); } catch (e) {}
+				}
+			}
 		}
 	} catch (e) {
 		ioStatus('test-io-status', t('Could not save the test script') + ': ' + e.message);
@@ -7150,6 +7172,8 @@ function testClone() {
 	if (!cur) return;
 	const copy = JSON.parse(JSON.stringify(cur));
 	delete copy.id;
+	delete copy._deleted;
+	delete copy._eventFieldsError;
 	copy.name = (copy.name || t('Script')) + ' (2)';
 	_testScripts.push(copy);
 	_testCurrentIdx = _testScripts.length - 1;
@@ -7161,11 +7185,16 @@ async function testDelete() {
 	const cur = testCurrent();
 	if (!cur) return;
 	if (!confirm(t('Delete this test script?'))) return;
+	// cancel a pending debounced write and mark the script: a queued or
+	// in-flight save must not resurrect it
+	if (_testSaveTimer) { clearTimeout(_testSaveTimer); _testSaveTimer = null; }
+	cur._deleted = true;
 	if (cur.id) {
 		try {
 			const resp = await pysimFetch('/api/test/scripts/delete', {id: cur.id});
 			if (resp.error) throw new Error(resp.error);
 		} catch (e) {
+			cur._deleted = false;   // the server still has it: keep it deletable
 			ioStatus('test-io-status', t('Could not delete the test script') + ': ' + e.message);
 			return;
 		}
@@ -7558,6 +7587,8 @@ function testStepCollect() {
 			p.event = val('test-f-evtype') || '03';
 			const raw = val('test-f-evfields');
 			p.fields = {};
+			// the flag reflects THIS collect, so fixing the JSON saves again
+			delete step._eventFieldsError;
 			if (raw) {
 				try {
 					const obj = JSON.parse(raw);
@@ -7567,7 +7598,6 @@ function testStepCollect() {
 					step._eventFieldsError = true;
 				}
 			}
-			if (!step._eventFieldsError && !raw) delete step._eventFieldsError;
 		}
 		else if (kind === 'file-write' || kind === 'file-read') {
 			p.path = val('test-f-path');