test(gamification): pin aggregate level to #11604 XP-derived semantics

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What was broken

tests/unit/gamification/aggregate-profile-3484.test.ts failed on release/v3.8.51 (and PR #11644's CI):

✖ #3484 getAggregateXp sums XP across keys and takes the highest level
  AssertionError: 2 !== 5   (agg.currentLevel)

Root cause

#11604 (fix(dashboard): keep Profile XP and level consistent) deliberately changed getAggregateXp() in src/lib/db/gamification.ts:

- SELECT COALESCE(SUM(total_xp), 0) AS total_xp,
-        COALESCE(MAX(current_level), 1) AS current_level,
+ SELECT COALESCE(SUM(total_xp), 0) AS total_xp,
   MAX(updated_at) AS updated_at
...
- currentLevel: row?.current_level ?? 1,
+ currentLevel: calculateLevel(totalXp),

The aggregate level must be derived from the same summed XP the profile displays. The #3484 test still asserted the old MAX(stored level) semantics — and its fixtures were off-curve anyway (100 XP → level 2 and 250 XP → level 5 are impossible on the XP curve where level 2 requires 282 XP).

Fix

tests/unit/gamification/aggregate-profile-3484.test.ts (lines 50–65):

• Test renamed: "sums XP across keys and takes the highest level" → "…derives the level from the summed XP".
• Fixtures aligned to the XP curve: key-a: 100 XP / level 1, key-b: 250 XP / level 1 (both individually level 1; sum 350 crosses the level-2 threshold of 282).
• Assertion: currentLevel === 2 (calculateLevel(350)), pinning the fix(dashboard): keep Profile XP and level consistent #11604 semantics.

Green evidence (after)

ℹ tests 4
ℹ pass 4
ℹ fail 0

Files: tests/unit/gamification/aggregate-profile-3484.test.ts, changelog.d/fixes/11604-aggregate-profile-level-pin.md.

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Superseded by consolidated PR #11679, which includes this fix alongside the other five failing-test root causes (per the requested PR organization: one PR for docs drifts #11673, one consolidated PR for all failing tests).