feat: enable index resumption and enforce directory creation

The command now supports reopening existing indexes instead of failing when the output file exists. Control flow branches between opening an existing index and constructing a new one, moving configuration setup exclusively to the creation path. Directory existence is enforced upfront with proper I/O error propagation. The --force flag retains its original semantics by removing the target directory before proceeding with a fresh build.

DevDocMD/implementation/partition_layer_cache.md

@@ -1,5 +1,21 @@
 # Partition and layer caching (discussion)
 
+**Superseded (2026-08-21):** `obikpartition` and `obilayeredmap` are no
+longer separate workspace crates — both were folded back into `obikindex`
+as submodules (`obikindex::partition`, `obikindex::layer`), alongside the
+crate's original content as `obikindex::index`, purely to reduce the
+crate count (no behavior change). Every mention of `obikpartition`/
+`obilayeredmap` as a *crate* below, and every dependency-direction
+argument phrased in terms of "which crate depends on which" (e.g. "this
+crate depends only on `obilayeredmap` and below, never on `obikindex`"),
+describes that now-superseded split-crate architecture and is kept as-is
+for historical context — read `obikpartition::X` as `obikindex::
+partition::X` and `obilayeredmap::X` as `obikindex::layer::X` throughout.
+The underlying module boundary and its rationale (Layer tier / Partition
+tier / Index tier, each depending only downward) are unchanged; only the
+crate-vs-module packaging changed. See [obikindex::layer](layer_tier.md)
+for the current module doc.
+
 Status (2026-08-20, latest pass): (1) done — `obilayeredmap::Layer`
 exists, `Mat` is gone. (1b) done — `Layer::Empty`, the first non-ready
 state, added (panics on every read method). (2a) done — the