feat(forge): Add FixedChargeLookupFailed error variant for charge lookup failures

Author: TKanX

src/forge/error.rs

@@ -52,6 +52,22 @@ pub enum Error {
         atom_index: usize,
     },
 
+    /// Fixed charge lookup failed (atom not found in `ffcharge` database).
+    ///
+    /// This typically indicates a mismatch between protonation state and
+    /// expected atoms, or an unsupported residue/atom name combination.
+    #[error("no {category} charge found: residue '{residue}' atom '{atom}' (position: {position})")]
+    FixedChargeLookupFailed {
+        /// Category of the fixed charge lookup (protein, nucleic, water, ion).
+        category: &'static str,
+        /// Residue name that was being looked up.
+        residue: String,
+        /// Atom name that was not found.
+        atom: String,
+        /// Position within chain (e.g., "NTerminal", "Middle").
+        position: String,
+    },
+
     /// Required force field parameter not found.
     ///
     /// Occurs when an atom type is assigned but no corresponding
@@ -110,6 +126,32 @@ impl Error {
         Self::MissingBioMetadata { atom_index }
     }
 
+    /// Creates a [`FixedChargeLookupFailed`](Error::FixedChargeLookupFailed) error.
+    ///
+    /// # Arguments
+    ///
+    /// * `category` — Category of the lookup (protein, nucleic, water, ion)
+    /// * `residue` — Residue name
+    /// * `atom` — Atom name
+    /// * `position` — Position within chain
+    ///
+    /// # Returns
+    ///
+    /// A [`FixedChargeLookupFailed`](Error::FixedChargeLookupFailed) error variant.
+    pub fn fixed_charge_lookup_failed(
+        category: &'static str,
+        residue: impl Into<String>,
+        atom: impl Into<String>,
+        position: impl Into<String>,
+    ) -> Self {
+        Self::FixedChargeLookupFailed {
+            category,
+            residue: residue.into(),
+            atom: atom.into(),
+            position: position.into(),
+        }
+    }
+
     /// Creates a [`MissingParameter`](Error::MissingParameter) error.
     ///
     /// # Arguments