Expand unit-test coverage of the STDP simulation core

[slavazeph-coder]

brainsnn-r3f-app/src/utils/sim.js

Vitest is already wired up (npm test, 22 tests in src/test/). The STDP update rule could use deeper coverage:

• weights strengthen when pre fires before post
• weights weaken when post fires before pre
• bursts increase activity on downstream regions
• resetState() returns the initial state shape
• weights stay clamped to their min/max bounds over long runs

Add the specs under src/test/ following the existing test style.