Build the Service Layer

public with sharing class MilestoneStatusService {
    private static final Map<String, Set<String>> ALLOWED_TRANSITIONS = new Map<String, Set<String>>{
        'Not Started' => new Set<String>{'In Progress'},
        'In Progress' => new Set<String>{'Complete', 'Blocked'},
        'Blocked' => new Set<String>{'In Progress'}
    };

    public static void validateStatusTransition(List<Milestone__c> newMilestones, Map<Id, Milestone__c> oldMap) {
        for (Milestone__c m : newMilestones) {
            String oldStatus = oldMap.get(m.Id).Status__c;
            if (oldStatus != m.Status__c && !ALLOWED_TRANSITIONS.get(oldStatus).contains(m.Status__c)) {
                m.addError('Cannot move a Milestone directly from ' + oldStatus + ' to ' + m.Status__c + '.');
            }
        }
    }
}

1. public with sharing class MilestoneStatusService { — the service class.
2. private static final Map<String, Set<String>> ALLOWED_TRANSITIONS = new Map<String, Set<String>>{ — a named constant, not scattered if conditions.
3. 'Not Started' => new Set<String>{'In Progress'}, — one entry per starting status.
4. 'In Progress' => new Set<String>{'Complete', 'Blocked'}, — allowed next statuses.
5. 'Blocked' => new Set<String>{'In Progress'} — allowed next statuses.
6. }; — closes the map literal.
7. (blank line)
8. public static void validateStatusTransition(List<Milestone__c> newMilestones, Map<Id, Milestone__c> oldMap) { — plain parameters, no trigger dependency.
9. for (Milestone__c m : newMilestones) { — iterates every updated Milestone.
10. String oldStatus = oldMap.get(m.Id).Status__c; — looks up the prior status.
11. if (oldStatus != m.Status__c && !ALLOWED_TRANSITIONS.get(oldStatus).contains(m.Status__c)) { — only checks transitions that actually changed, against the rules table.
12. m.addError('Cannot move a Milestone directly from ' + oldStatus + ' to ' + m.Status__c + '.'); — blocks the record with a specific message.
13. } — closes the if.
14. } — closes the for loop.
15. } — closes the method.
16. } — closes the class.

A Map<String, Set<String>> of allowed transitions is a small, self-contained rules table — readable, testable, and easy to extend without touching the trigger or handler at all.

public with sharing class ProjectHealthService {
    private MilestoneSelector milestoneSelector;

    public ProjectHealthService(MilestoneSelector milestoneSelector) {
        this.milestoneSelector = milestoneSelector;
    }

    public ProjectHealthService() {
        this(new MilestoneSelector());
    }

    public Map<Id, String> calculateHealth(Set<Id> projectIds) {
        List<Milestone__c> milestones = milestoneSelector.selectByProjectIds(projectIds);
        // groups by Project__c, checks for any overdue Milestone, returns 'At Risk' or 'On Track' per project
        return new Map<Id, String>(); // full grouping logic omitted for brevity
    }
}

1. public with sharing class ProjectHealthService { — the service class.
2. private MilestoneSelector milestoneSelector; — a field, not a local variable — the injected dependency.
3. (blank line)
4. public ProjectHealthService(MilestoneSelector milestoneSelector) { — a constructor that accepts the dependency.
5. this.milestoneSelector = milestoneSelector; — stores whatever was passed in.
6. } — closes that constructor.
7. (blank line)
8. public ProjectHealthService() { — a second, no-argument constructor.
9. this(new MilestoneSelector()); — delegates to the first constructor with a real selector.
10. } — closes the no-argument constructor.
11. (blank line)
12. public Map<Id, String> calculateHealth(Set<Id> projectIds) { — the method signature.
13. List<Milestone__c> milestones = milestoneSelector.selectByProjectIds(projectIds); — uses this.milestoneSelector, whichever one was injected.
14. // groups by Project__c, checks for any overdue Milestone, returns 'At Risk' or 'On Track' per project — the actual logic, summarized.
15. return new Map<Id, String>(); // full grouping logic omitted for brevity — a placeholder return.
16. } — closes the method.
17. } — closes the class.

Module 43\'s exact dependency-injection shape — a test can call new ProjectHealthService(fakeMilestoneSelector) to supply hand-built Milestone data without any real DML, testing the health-calculation logic in complete isolation.

If different project types eventually need different health rules (a Fixed-Price project might define "at risk" differently than a Time-and-Materials one), an IProjectHealthRule interface — Module 43\'s exact strategy pattern — would let ProjectHealthService accept a rule strategy via its constructor, selected by a factory based on project type, without changing ProjectHealthService itself.