🧪 Add tests for generateIslandGrid engine function

[Ap-0007]

🎯 What: The testing gap addressed
This PR addresses the lack of tests for the complex island generation algorithm (generateIslandGrid) in lib/island-engine.ts. This pure function requires deterministic logic to convert 30-day activity arrays into thematic grid arrays, and having solid tests helps prevent regressions when tweaking formulas.

📊 Coverage: What scenarios are now tested
Tests have been added utilizing Node.js' native test runner (node:test). The test suite covers:

• Generating the grid with default parameters.
• Handling empty activity arrays gracefully.
• Correctly condensing a 30-day activity array into 10 columns by checking averages.
• Properly assigning the "hot streak" flag only to the max activity tile if the commit count is >= 5.
• Confirming that correct themes (e.g. "spooky") emit the correct theme emojis into the tiles.

✨ Result: The improvement in test coverage
The critical domain logic responsible for determining user island representation is now fully covered by targeted unit tests, ensuring future refactors won't inadvertently break streak logic, theme mappings, or layout calculations.

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PR created automatically by Jules for task 13831825439870422474 started by @Ap-0007

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Project	Deployment	Actions	Updated (UTC)
side_pro	Ready	Preview, Comment	May 18, 2026 1:09pm