Phase 15: snapshots, forks, export/import, cognitive replay, experiments

- snapshots: checksummed JSON archive of every mind-scoped state table plus
  identity/personality/config and the event position; stored, downloadable
- fork: restore into a new mind with all ids remapped; event references are
  kept so history before the fork is shared by reference
- export/import: same archive; import is hostile-input validated (size,
  allow-listed tables, row caps, column names, checksum, forced mind_id)
- compare: deterministic counts, belief diffs by proposition, personality
  and config diffs, event mix
- replay: Runtime.Step for synchronous drivers; sandbox fork driven one
  stimulus at a time under a forbidden capability context; recorded model
  responses via a replaying router; steps labelled exact / approximate /
  diverged with recorded-vs-replayed digests
- experiments: definition (overrides, stimuli, expectations, seed, duration),
  background run in a disabled subject mind, stored and exportable results
- migration 0015, API, UI (Snapshots and Replay tabs, Experiments section,
  compare page), noemactl snapshots/compare/replay/experiments, docs

web/templates/layout.html

@@ -53,6 +53,9 @@
   <a href="/integrations" {{if hasPrefix .Path "/integrations"}}aria-current="page"{{end}}>Integrations</a>
   <a href="/policies" {{if hasPrefix .Path "/policies"}}aria-current="page"{{end}}>Policies</a>
 </div>
+<div class="navgroup"><div class="navhead">Study</div>
+  <a href="/experiments" {{if hasPrefix .Path "/experiments"}}aria-current="page"{{end}}>Experiments</a>
+</div>
 <div class="navgroup"><div class="navhead">System</div>
   <a href="/system" {{if eq .Path "/system"}}aria-current="page"{{end}}>System</a>
   <a href="/audit" {{if eq .Path "/audit"}}aria-current="page"{{end}}>Audit</a>