A single malformed alarm entry (bad/missing id, wrong type) used to
discard the ENTIRE persisted list on next load, and a fully unparseable
payload let the periodic recalculation silently overwrite it with an
empty one -- both destroyed valid alarms with no user action.
Adds a shared per-entry tolerant-parse helper (persistencia_tolerante.dart)
that skips and logs only the bad entry; ServicioAlarmas now normalizes its
cached raw after a partial load (no dirty-guard thrash) and sets a
degraded-read flag after a total decode failure that suppresses automatic
writes until a good read or an explicit user mutation restores authority.
