Add optimistic revisions, keep dirty state on failed saves, quarantine corrupt data

Phase 1 of the improvement plan (PR 4 of the sequence): stop concurrent
whole-object saves from losing edits, stop failed saves from reporting clean,
and stop corrupt user data from silently vanishing.

Revisions (core/schema.js, core/store.js):
- Every guide and step carries a monotonic `revision`, bumped on each store
  write. Legacy v1 data without the field reads as revision 0 and upgrades on
  its next save — no migration pass, no data rewrite.
- saveGuide/saveStep accept { expectedRevision } for compare-and-swap saves;
  a mismatch throws RevisionConflictError instead of clobbering. Direct user
  edits pass no expectation (the user is the authority); background writers
  must pass one.

Stale AI responses (app/text-intel.js):
- generateStepPatch snapshots the step revision before the (slow) model call,
  re-reads the step after it, and saves with the original expectedRevision. A
  user edit made during generation now surfaces as "the step changed while AI
  was generating; nothing was overwritten" — previously the AI response
  silently overwrote the newer edit.

Autosave truthfulness (app/renderer/editor.js):
- flushStep/flushGuide cleared the dirty flag BEFORE awaiting the IPC save,
  so a rejected save (invoked via a debounce that never handled rejections)
  lost the visible dirty state. The flag is now cleared only after a durable
  save; failures keep it dirty, surface a persistent saveError in editor
  meta, toast the user, and retry on the next edit or explicit save.
- Navigating away from the editor flushes pending debounced saves so the
  last edit can never be dropped by a view switch.

Corruption quarantine (core/store.js):
- listGuides/listSteps used to silently skip unreadable entries — a corrupt
  guide just vanished from the library. Corrupt guide/step directories are
  now moved to library/quarantine (original bytes preserved) and recorded in
  a recovery report (store.getRecoveryReport()) for the UI. Empty in-progress
  directories are still skipped quietly — absence of guide.json is not
  corruption.

Tests: revision increments, stale-CAS rejection with user edit surviving,
CAS success path, guide CAS, v1 no-revision upgrade, guide/step quarantine
with preserved bytes + recovery report, empty-dir non-quarantine, AI
stale-write rejection end-to-end (user edit mid-generation survives) and the
clean-apply path. 240 unit tests pass; startup smoke and sample-artifact
E2E pass.

Co-Authored-By: Claude Fable 5 <[email protected]>

app/text-intel.js

@@ -624,6 +624,10 @@ public static class Win32 {
       if (!guide || !step) {
         return { ok: false, reason: 'Guide or step not found.' };
       }
+      // Snapshot the revision now: AI generation is slow, and the user may
+      // edit the step meanwhile. We save with this expectedRevision so a
+      // response built from stale data cannot overwrite a newer user edit.
+      const baseRevision = Number.isInteger(step.revision) ? step.revision : 0;
 
       const currentBlock = blockId
         ? [...(step.textBlocks || []), ...(step.codeBlocks || []), ...(step.tableBlocks || [])].find((b) => b.id === blockId) || null
@@ -716,8 +720,26 @@ public static class Win32 {
         guideId,
       });
       const patch = normalizeAiPatch(raw);
-      const updated = applyAiPatchToStep(step, patch, { target, blockId });
-      const saved = this.store.saveStep(guideId, updated);
+      // Re-read the step: while generation ran, a capture auto-doc or another
+      // background write may have advanced it. Apply the patch to the current
+      // step and save with the original expected revision so a user edit made
+      // during generation causes a conflict instead of a silent overwrite.
+      let currentStep = step;
+      try {
+        currentStep = this.store.getStep(guideId, stepId) || step;
+      } catch {
+        currentStep = step;
+      }
+      const updated = applyAiPatchToStep(currentStep, patch, { target, blockId });
+      let saved;
+      try {
+        saved = this.store.saveStep(guideId, updated, { expectedRevision: baseRevision });
+      } catch (err) {
+        if (err && err.code === 'STEPFORGE_REVISION_CONFLICT') {
+          return { ok: false, reason: 'The step changed while AI was generating; nothing was overwritten.' };
+        }
+        throw err;
+      }
       return { ok: true, step: saved, patch };
     } catch (err) {
       return { ok: false, reason: err && err.message ? err.message : 'AI generation failed.' };