MR-925: validation-prototypes scaffolding + exp 1.1 + exp 1.2

- exclude validation-prototypes/ and merge-insert-cas-repro from the main
  workspace so the nested cargo workspace can use its own pin set
- add validation-prototypes/{factorized-batches,custom-lance-index}/
  scratch crates (never merged to main; long-lived branch only)
- exp 1.1 — factorized batches through DataFusion ops: writeup at
  .context/experiments/factorized-batches.md (5 cells × 8 ops; all
  scalar-keyed ops accept List<UInt64> input, UNNEST via CROSS JOIN
  fails in DF 52.5)
- exp 1.2 — custom Lance index plugin from outside lance: writeup at
  .context/experiments/custom-lance-index.md (5 probes; transaction
  surface is open, SCALAR_INDEX_PLUGIN_REGISTRY is closed → hard
  blocker for MR-737 §5.4; recommends upstream path or external-index
  path)

Cargo.toml

@@ -6,6 +6,13 @@ members = [
     "crates/omnigraph-cli",
     "crates/omnigraph-server",
 ]
+exclude = [
+    # MR-925 / MR-737 pre-Phase-0 validation prototypes — nested cargo
+    # workspace; never merged to main.
+    "validation-prototypes",
+    # Existing scratch crate kept out of the main workspace.
+    ".context/scratch/merge-insert-cas-repro",
+]
 default-members = [
     "crates/omnigraph",
     "crates/omnigraph-cli",