refactor: rename obikindex to obikindexer

DevDocMD/implementation/partition_layer_cache.md

@@ -16,6 +16,22 @@ 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.
 
+**Superseded, second event, same day (2026-08-21):** `obikpartitionner`
+and `obikderep`, the two algorithm crates, are also gone — but unlike
+`obikpartition`/`obilayeredmap` above, they were **not** folded into
+`obikindex`. They were first (mistakenly) merged into `obikindex` as an
+`algorithms` submodule, then corrected into a new sibling crate,
+**`obikindexer`**, holding `obikindexer::algorithms::{partitionner,
+dereplicator}` and depending on `obikindex` — never the reverse, same
+dependency direction `obikpartitionner`/`obikderep` already had. Read
+`obikpartitionner::X` as `obikindexer::algorithms::partitionner::X` and
+`obikderep::X` as `obikindexer::algorithms::dereplicator::X` throughout
+what follows. The distinction the mistake surfaced, worth keeping: data
+crates (`obikindex`, holding the `index`/`partition`/`layer` model) merge
+naturally into one crate as submodules; algorithm crates that operate on
+that model from outside stay separate, so the dependency only ever runs
+one way.
+
 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