⬆️ version bump to v4.5

- Update obioptions.Version from "Release 4.4.29" to "/v/ Release v5"
- Update version.txt from 4.29 → .30
(automated by Makefile)

autodoc/docmd/pkg/obikmer/kdi_writer.md

@@ -0,0 +1,38 @@
+# KDI File Format and Writer
+
+The `obikmer` package implements a compact, sorted sequence storage format for 64-bit k-mers using delta encoding and sparse indexing.
+
+## Core Format (`.kdi`)
+
+- **Magic header**: `KDI\x01` (`4 bytes`) identifies the file type.
+- **Count field**: `uint64 LE`, total number of k-mers (patched at close).
+- **First value**: `uint64 LE`, the initial k-mer stored as an absolute integer.
+- **Deltas**: Subsequent values encoded via *delta-varint* (difference from previous k-mer), enabling high compression for sorted sequences.
+
+## Writer (`KdiWriter`)
+
+- **Strict ordering**: K-mers must be written in *strictly increasing order*.
+- Efficient buffering via `bufio.Writer` (64 KB buffer).
+- Internally tracks:
+  - Current k-mer count,
+  - Previous value (for delta computation),
+  - Bytes written in data section.
+- **Sparse indexing**: Every `defaultKdxStride` k-mers, an entry is recorded in memory for random access.
+
+## Companion Index (`.kdx`)
+
+- Written automatically on `Close()` if indexing entries exist.
+- Stores `(kmer, file_offset)` pairs for fast seek-to-position lookups (e.g., binary search on k-mer range).
+- Enables efficient random access without full file scan.
+
+## Usage Pattern
+
+```go
+w, _ := obikmer.NewKdiWriter("data.kdi")
+for _, kmer := range sortedKMers {
+    w.Write(kmer)
+}
+w.Close()  // finalizes header, writes .kdx index
+```
+
+The format is optimized for memory-efficient storage and fast retrieval of sorted uint64 k-mers in genomic or sequence analysis pipelines.