Extract dereplication logic into obikderep crate and centralize paths

Move partition dereplication logic into a dedicated `obikderep` crate that implements a two-phase hash-split/merge strategy with Rayon for parallel processing. Centralize superkmer file path construction in `obilayeredmap` and update dependent crates to use the new pipeline and shared path helpers. Adjust test suites to explicitly invoke the dereplication step.

src/Cargo.lock

@@ -1515,6 +1515,24 @@ dependencies = [
  "xxhash-rust",
 ]
 
+[[package]]
+name = "obikderep"
+version = "0.1.0"
+dependencies = [
+ "niffler",
+ "obikindex",
+ "obikrope",
+ "obikseq",
+ "obilayeredmap",
+ "obiskbuilder",
+ "obiskio",
+ "obisys",
+ "rayon",
+ "sysinfo",
+ "tempfile",
+ "tracing",
+]
+
 [[package]]
 name = "obikentropy"
 version = "0.1.0"
@@ -1565,6 +1583,7 @@ dependencies = [
  "kodama",
  "obidebruinj",
  "obifastwrite",
+ "obikderep",
  "obikindex",
  "obikpartitionner",
  "obikphylo",
@@ -1607,6 +1626,7 @@ dependencies = [
  "memmap2",
  "niffler",
  "obicompactvec",
+ "obikderep",
  "obikindex",
  "obikrope",
  "obikseq",
@@ -1632,6 +1652,7 @@ dependencies = [
  "memmap2",
  "ndarray",
  "obicompactvec",
+ "obikderep",
  "obikindex",
  "obikpartitionner",
  "obikseq",