⬆️ 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/kdx.md

@@ -0,0 +1,29 @@
+# KDX Index Format and Functionality
+
+The `obikmer` package provides a sparse indexing mechanism for `.kdi` files (likely storing sorted k-mers with delta encoding). The **`.kdx` file** serves as a fast lookup table to accelerate k-mer searches.
+
+## Core Concepts
+
+- **Magic bytes**: `KDX\x01` validates file integrity.
+- **Stride-based sparsity**: One index entry every *N* k-mers (default: 4096), balancing memory vs. search speed.
+- **Entry structure**: Each entry stores:
+  - `kmer`: the k-mer value at that index position.
+  - `offset`: absolute byte offset in the corresponding `.kdi` file.
+
+## Key Operations
+
+- **Loading**: `LoadKdxIndex()` reads and validates a `.kdx` file; returns `(nil, nil)` if missing (graceful degradation).
+- **Searching**: `FindOffset(target uint64)` performs binary search over index entries to find the *best jump point*:
+  - Returns `offset`, `skipCount` (k-mer count already passed), and a boolean success flag.
+  - Enables efficient seeking: after `offset`, only up to *stride* k-mers need linear scanning.
+- **Writing**: `WriteKdxIndex()` serializes an in-memory index to disk (for building indexes).
+- **Helper**: `KdxPathForKdi()` derives the `.kdx` path from a given `.kdi` file.
+
+## Performance
+
+- Search complexity: **O(log M)** for the binary search (where *M* = #index entries), plus ≤ stride linear steps.
+- Memory footprint: Linear in index size (16 bytes per entry), highly scalable for large k-mer sets.
+
+## Design Philosophy
+
+Minimalist, binary-safe format with explicit endianness (little-endian), no external dependencies beyond `encoding/binary`, and robust error handling.