test(edge): edge cases and regression tests

[wazolab]

Parent

Part of the Testing & Code Quality milestone. Related to #30.

Description

Add edge case and regression tests to harden the library against unexpected inputs and state transitions. These tests prevent future regressions as the codebase evolves.

Edge cases to cover

Case	Expected behavior
Empty feature collection (no features from source)	No markers created, no errors, stale markers cleaned up
Non-Point geometry feature	`console.error` logged, feature skipped, rendering continues
Cluster with 0 leaves (empty cluster)	`console.error` logged, cluster skipped
`loadClusterLeaves` rejection (source error)	Returns `false`, logs warning, rendering aborted gracefully
Feature with duplicate IDs in source	Last feature wins in `featuresMap`, no crash
Rapid consecutive `moveend` events	Only the final render completes (abort counter works)
`onRemove` called before `onAdd`	No-op, no error
`setSelectedFeature` called before `onAdd`	Pin marker placed at coordinates (no map interaction)
`resetSelectedFeature` when no selection exists	No-op, no error
Cluster with leaves exactly at `unfoldedClusterMaxLeaves`	Renders as unfolded (boundary condition)
Zoom exactly at `clusterMaxZoom`	All clusters unfold regardless of leaf count
Zoom exactly at `clusterMinZoom`	Clusters start unfolding
`getFeatureId` with `properties.id = 0`	Returns `"0"`, not falsy
`getFeatureId` with neither metadata.id nor properties.id	Currently throws TypeError — document and/or fix
`deepMerge` with `null` values in source	Should replace target value with `null`

Acceptance Criteria

• All edge cases listed above have corresponding tests
• No test relies on implementation details (test behavior, not internals)
• Each edge case test is documented with a comment explaining the scenario
• `yarn test` passes
• `yarn coverage` shows meaningful improvement

Files Likely Involved

• `tests/edge-cases.test.ts` (new)
• `tests/utils/deep-merge.test.ts` (extend)
• `tests/utils/index.test.ts` (extend)

Dependencies

• Blocked by: refactor(test): build comprehensive MapLibre mock #64 (mock), test(utils): unit tests for pure utility functions #62 (utility tests), test(core): renderCustom, feature selection and event dispatch tests #67 (core tests)
• Blocks: none

Parallelizable

no — final tier, builds on all prior test infrastructure