Unify timestamp checks

[elichad]

Alternative to #56 focused only on unifying the approach to timestamp validation. Adds a combined check into the new file 3_timestamp_format.ttl as I noticed we haven't used the index 3 for any other checks. Example error message:

     [ five-safes-crate-0.4_23 ]: Timestamp Format                                                                                    
                                                                                                                                      
      Timestamps MUST follow the RFC 3339 standard (YYYY-MM-DD'T'hh:mm:ss[.fraction](Z | ±hh:mm)).                                    
                                                                                                                                      
          Failed checks                                                                                                               
                                                                                                                                      
       [ five-safes-crate-0.4_23.0 ] Timestamp Format:                                                                                
                                      Timestamps MUST follow the RFC 3339 standard (YYYY-MM-DD'T'hh:mm:ss[.fraction](Z | ±hh:mm)).    
         Detected issues                                                                                                              
         - [Violation"01 Jan 2026 10:00"  on <01 Jan 2026 10:00>]: All `startTime` and `endTime` values MUST follow the RFC           
         3339 standard (YYYY-MM-DD'T'hh:mm:ss[.fraction](Z | ±hh:mm)).                                                                
                                                                              

Downside: the error messages don't state which entity the problem timestamp is from. But I think overall it's cleaner than the option presented in #56/#50

[elichad]

This approach could also be incorporated into the generic "Review Process" check set that we were considering making (i.e. checks that are applicable to all the different phases)

[douglowe]

I think this is an elegant solution. There's a couple places we have redundant code now, which could be removed.

[douglowe]

This all looks good to me now.