Function add_hydrogens 

pub fn add_hydrogens(
    structure: &mut Structure,
    config: &HydroConfig,
) -> Result<(), Error>

Adds hydrogens to all standard residues in-place, updating protonation states when needed.

This function implements a multi-phase pipeline:

1. Disulfide detection — Identifies CYS pairs forming S-S bonds and relabels to CYX.
2. Non-HIS protonation — Applies pKa-based titration to ASP, GLU, LYS, ARG, CYS, TYR (only when target_ph is specified).
3. HIS protonation — Determines HIS state via pH thresholds, salt bridge detection, and tautomer strategy.
4. Hydrogen construction — Builds hydrogens according to template geometry and terminal-specific rules.

Arguments

• structure - Mutable structure whose residues will be protonated and hydrated.
• config - Hydrogenation configuration controlling pH, strategy, and options.

Returns

Ok(()) when hydrogenation succeeds.

Errors

Returns Error::MissingInternalTemplate when no template is found or Error::IncompleteResidueForHydro when required anchor atoms are missing.