Fix locality age method persistence and scoped validation

[Copilot]

Switching between dating methods (Time unit, Absolute, Composite) in the Locality Age tab was clearing previously entered age values, forcing users to re-enter data when exploring different dating approaches.

Changes

• Age draft persistence: AgeTab now maintains method-specific state (ageDraftsByMethod) that preserves min_age, max_age, and basis field selections per dating method. Switching methods saves current values and restores previously entered values for the target method.

• Scoped validation: Validation logic refactored to evaluate required basis fields only against the active dating method. For composite mode, validation enforces that one row uses time_unit basis while the other uses absolute basis, determined by which fields are populated.

• Test coverage: Added regression tests for method round-trip scenarios and fraction-based age recalculation.

Example

// Before: switching methods cleared all age values
// After: preserved per-method in ageDraftsByMethod state
const handleDateMethChange = (value: LocalityDatingMethod) => {
  const nextDrafts = { ...ageDraftsByMethod }
  
  // Save current values before switch
  if (currentDateMethod) {
    nextDrafts[currentDateMethod] = getAgeFields(editData)
  }
  
  // Restore saved values for target method
  const restoredFields = nextDrafts[nextDateMethod] ?? emptyAgeFields()
  setEditData({ ...editData, date_meth: nextDateMethod, ...restoredFields })
}

Documentation

Updated README.md and CHANGELOG.md with reviewer notes on the persistence behavior and validation scoping.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.