Docs: Create ARCHITECTURE.md

[niyazmft]

Fixes #80

Adds comprehensive architectural documentation for Emberfall, including sections on the visual 2.5D system, deterministic math, FSMs, and the updated GdUnit4 testing methodology.

[gemini-code-assist]

Code Review

This pull request integrates the GdUnit4 unit testing framework into the project, replacing legacy shell-based testing with robust test suites, runners, and assertions. The code review identified several critical issues in the newly added framework files: potential runtime crashes in GdObjects.gd and GdUnitSceneRunnerImpl.gd due to missing validity checks on freed objects and loaded scenes; a redundant directory opening and potential null dereference in GdUnitFileAccess.gd; a strict typing mismatch in GdScriptParser.gd where a base Token is returned instead of a Variable; and a logic gap in GdUnitTestSessionRunner.gd where tests are executed even if the session startup hook fails.

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