feat: introduce obitaxonomy crate for hierarchical taxonomy parsing

Adds the `obitaxonomy` crate to parse and validate hierarchical taxonomy paths using a strict `taxonomy:/name@rank/...` syntax. Replaces generic string-based path matching in predicates with structured `TaxPath` and `TaxPattern` types, enforcing explicit anchor constraints and rank-aware semantics. Updates filtering documentation to clarify optional leading slashes and segment-boundary matching rules.

src/obitaxonomy/src/error.rs

@@ -0,0 +1,38 @@
+use std::fmt;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum TaxError {
+    /// Stored value does not start with the `taxonomy:/` prefix.
+    MissingPrefix,
+    /// Stored path contains no segments after the prefix.
+    EmptyPath,
+    /// Query pattern contains no segments (after stripping anchors).
+    EmptyPattern,
+    /// A segment has an empty name (e.g. consecutive `/`).
+    EmptySegmentName,
+    /// A segment has a trailing `@` with no rank name.
+    EmptyRankName { segment: String },
+    /// A segment contains more than one `@`.
+    AmbiguousRank { segment: String },
+}
+
+impl fmt::Display for TaxError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            TaxError::MissingPrefix =>
+                write!(f, "taxonomy path must start with \"taxonomy:/\""),
+            TaxError::EmptyPath =>
+                write!(f, "taxonomy path has no segments"),
+            TaxError::EmptyPattern =>
+                write!(f, "taxonomy query pattern has no segments"),
+            TaxError::EmptySegmentName =>
+                write!(f, "segment has an empty name"),
+            TaxError::EmptyRankName { segment } =>
+                write!(f, "segment has '@' with no rank name: {segment:?}"),
+            TaxError::AmbiguousRank { segment } =>
+                write!(f, "segment contains more than one '@': {segment:?}"),
+        }
+    }
+}
+
+impl std::error::Error for TaxError {}