test(map): cover cluster→task navigation guard in MapScreen

[devOgazi]

Summary

Closes #147

MapScreen renders markers and cluster markers, but the most user-critical map interaction — tapping a marker/callout to reach a task — was only partially tested. MapScreen.test.tsx already covered single-task marker navigation, yet the cluster guard behavior was unverified:

• a multi-task cluster must not navigate (no zoom/expand path in Jest), and
• a count:null promoted single marker (an isolated task bucketed as one) must navigate correctly.

This extends the existing file with the missing guard tests, using the Marker mock that exposes onCalloutPress on its component instance.

What's covered (new tests)

• Multi-task cluster → no navigation. 11 tasks in the same grid bucket collapse into one cluster (count > 1). Pressing its callout asserts navigate is never called — verifying the if (!isCluster && firstTask) guard prevents navigation for multi-task clusters (the "tap a cluster, nothing happens because zoom isn't supported here" path).
• count:null promoted single marker → navigates. 10 tasks collapse into one multi-task cluster while a lone task in a separate grid bucket is promoted to a single marker (count === null). Pressing its callout navigates to TaskDetail with the correct taskId.
• The pre-existing single-task marker navigation test is retained.

The existing MapScreenPerformance.test.tsx continues to benchmark clustering only; correctness of the expansion path now lives here.

Approach

• Mocks react-native-maps so Marker is a plain View, useTaskFeed, useLocation, and @react-navigation/native (asserting navigate('TaskDetail', { taskId })).
• Triggers onCalloutPress directly on the Marker test instance, which exercises the real guard in handleCalloutPress / MapScreen.tsx:250-254.
• Builds task sets at deliberately separated coordinates so clusterTasks produces the exact mix of multi-task cluster + promoted single marker needed to hit both branches.

Verification

• npm test → 416/416 passing (45 suites); the MapScreen suite is 10/10.
• npm run typecheck (tsc --noEmit) → clean.
• npm run lint → 0 errors on the file.

Out of scope

Zoom-to-expand gesture behavior (platform-native, untestable in Jest) and a cluster bottom-sheet (separate UX issue).