docs: repoint architecture docs at the workspace docs tree; ignore .scratch/

The workspace-root scratch cleanup (2026-09-27) deleted .repro/, the _review_*/
probe bundles and _probe_dataarea/. These tracked documents cited files inside
those directories, so their references are now explicitly marked as historical
and, where the evidence survived, repointed:

- record-channel.md section 7: the ten reproduction conditions are recorded in the
  archived review (docs/archive/reviews/phase12-review.md); reproduce.py is gone.
- studio-3d-stages.md: the _review_scene3d_20260914 review is archived under
  docs/archive/reviews/; the probe_stage6_* scripts and the .repro/ scan scripts
  are marked deleted. The perf report's stale .repro package path is explained in
  a note instead of editing the committed probe report, which feeds
  compile_scene.py --lock-from-probe.
- studio-architecture.md: the scan scripts that generated the diagrams are gone;
  the committed .svg/.png remain the surviving artefacts.

Also ignore .scratch/, the new per-task scratch directory (see
D:\orisys_ws\AGENTS.md). No source file is touched by this commit.

.gitignore

@@ -36,3 +36,6 @@ _parked_stale_assets/
 *.swp
 Thumbs.db
 .DS_Store
+
+# Agent scratch: one directory per task, see D:\orisys_ws\AGENTS.md
+.scratch/