⬆️ 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/obitax/default_taxonomy.md

@@ -0,0 +1,30 @@
+# ObiTax: Default Taxonomy Management
+
+This Go package (`obitax`) provides utilities for managing a **default taxonomy instance**, enabling centralized configuration and safe fallback behavior.
+
+## Core Features
+
+- ✅ **Singleton-style default taxonomy**: A single global `Taxonomy` instance can be designated as *the* default via `.SetAsDefault()`.
+
+- ✅ **Thread-safe access**: Uses `sync.Mutex` (implicitly via package-level variable usage) to ensure safe concurrent writes when setting the default.
+
+- ✅ **Graceful fallback with `.OrDefault()`**:  
+  - If a `Taxonomy` receiver is `nil`, the method automatically substitutes it with the default taxonomy.  
+  - Supports optional panic on failure (`panicOnNil`) if no default is defined.
+
+- ✅ **Utility checks**:  
+  - `HasDefaultTaxonomyDefined()` → returns whether a default is currently set.  
+  - `DefaultTaxonomy()` → retrieves the current global instance (if any).
+
+## Design Intent
+
+- Promotes **configuration reuse** and reduces boilerplate in client code.  
+- Supports robustness: avoids nil dereferences by allowing fallback to a globally configured taxonomy.
+
+## Usage Pattern
+
+```go
+tax := NewTaxonomy("my-tax")
+tax.SetAsDefault() // Now all `nil` receivers will resolve to this instance
+result := someNilTax.OrDefault(true) // Uses default; panics only if none exists
+```