fix: compute DiceRoll success count from fresh roll, not stale result (Closes #16)

The DiceRoll.roll() method was counting successes from `this.result`
(the stale/empty list from before rolling) instead of the newly rolled
values. Fixed by capturing the new roll in a local variable before
passing it to copy(). Added 6 unit tests covering the fix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.plans/issue-16-fix-diceroll-bug.md

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+# Plan: Fix DiceRoll Success Counting Bug (Issue #16)
+
+## Summary
+The `DiceRoll.roll()` method computes `numberOfSuccesses` from the stale `this.result` list instead of the freshly rolled values. The fix is to capture the new roll in a local variable before passing it to `copy()`, so both `result` and `numberOfSuccesses` reference the same fresh data.
+
+## Steps
+
+### Step 1: Fix `Dice.kt`
+**File**: `sharedUI/src/commonMain/kotlin/org/shahondin1624/lib/functions/Dice.kt`
+
+Change the `roll()` method to:
+1. Roll dice into a local `val newResult`
+2. Count successes from `newResult`
+3. Pass both to `copy()`
+
+### Step 2: Write unit test
+**File**: `sharedUI/src/commonTest/kotlin/org/shahondin1624/DiceTest.kt`
+
+Create a new test file with:
+- Test that after `roll()`, `numberOfSuccesses` equals the count of values >= threshold in `result`
+- Test with default threshold (5) and custom threshold
+- Test that `result` list has correct size (matches `numberOfDice`)
+- Test the `countSuccesses` extension function directly
+
+## AC Verification Checklist
+1. [ ] `numberOfSuccesses` is computed from newly rolled values (not stale result list)
+2. [ ] Unit test verifying correct success count after `.roll()`