fix(persist): one shared z-order across Source + WebcamSceneConfig rows

A webcam dragged between sources reverted to the bottom of the stack on
every relaunch: Source and WebcamSceneConfig each carried independent
per-type SortOrder counters, and Load appended all Sources before all
configs. Save now stamps both tables' SortOrder from the element's index
within scene.Elements; Load merges the two tables' rows by that shared z
(sources-first tie-break preserves legacy rows). Cross-type reorder now
survives a fresh LayoutStore reload.

Task 37 queued: defaults vs current layout split (creator directive) —
capture out-of-scope work in TASKS.md rather than folding it in.

TASKS/task-37-defaults-current-split.md

@@ -0,0 +1,48 @@
+# TASK 37 — Defaults vs current layout split
+
+**Status:** ☐ Queued (2026-09-20)
+
+## Idea (creator directive 2026-09-20)
+
+The app establishes a DB of static, branded layout objects as its defaults. Today the DB is BOTH the
+defaults and the live workspace: `LayoutStore.Save` deletes and reinserts the whole layout on every
+save, so "your last edit" and "the defaults" occupy the same rows. The recurring "reorder doesn't
+persist" bug was a symptom of that conflation.
+
+Store TWO data sets:
+
+- **`default`** — the established DB (branded factory scenes/layers), never mutated by user edits.
+- **`current`** — whatever the user currently has, stamped with the build-id.
+
+Load rule: honor `current` **iff** its build-id equals the running build's id (`BuildStamp.Id`),
+otherwise load `default`. Bonus: one-click "revert to defaults" = drop `current`.
+
+Two implementations were floated (config file vs second rows in the same DB); nothing decided — both
+are OPS acceptable to the creator. The proposed conceptual shape:
+
+```
+scene[0-4].layerList.[default | current].elementList
+```
+
+## Notes / facts established while investigating
+
+- `SceneCatalog` only defines the five canonical scene **names**; the actual default element set
+  exists only as whatever the DB holds at first migration. There is no standalone in-code factory
+  element set — "defaults" at migration time = a snapshot of today's DB.
+- SortOrder is a DB column only (not on models); `Source` and `WebcamSceneConfig` were stamped with
+  independent per-type sort counters and load appended all sources before all configs. That
+  cross-table order bug is ALREADY FIXED (2026-09-20, unified z-space, see commit) — TASK 37 is the
+  defaults/current data split on top of it.
+- `MainViewModel.LoadLayout` (MainViewModel.cs:348) seeds `SceneCatalog.All` when `Scenes.Count == 0`
+  and SaveLayoutNow deletes/reinserts everything (LayoutStore.Save.cs). Startup path:
+  `%APPDATA%\ytLlive\ytLlive.db` (MainViewModel.cs:332), `LayoutPathOverride` test seam.
+
+## Acceptance
+
+- A saved `current` layout with a matching build-id restores on start.
+- A `current` stamped by a different (older/newer) build-id falls back to `default`.
+- Revert-to-default restores `default` for one scene or all.
+
+## Cross-references
+
+- `ai.md` — persistence architecture; `MyMistakes.md` — prior reorder persistence failures.