99 Club Studio v1.31.3 — Random Pack mixed difficulty

Adds weighted mixed difficulty to Random compatible games & puzzles without changing the manual game-selection workflow.

What changed

Random Pack difficulty now supports Mixed — weighted alongside Easy, Standard and Challenge.
Default mixed profile is 25% Easy / 50% Standard / 25% Challenge.
Adds four quick presets:
- Balanced — 25 / 50 / 25
- Gentle — 50 / 40 / 10
- Mostly standard — 15 / 70 / 15
- Challenge-heavy — 10 / 40 / 50
Easy and Challenge percentages can be edited directly; Standard is automatically calculated as the remaining percentage.
Mixed difficulty is allocated as a pack-wide quota, not as unrelated random rolls. For example, an 8-activity pack at 25/50/25 produces 2 Easy, 4 Standard and 2 Challenge activities, then shuffles their positions.
Quotas use largest-remainder rounding for pack sizes that do not divide cleanly.
The difficulty plan remains deterministic for a given seed.
If an engine does not support the requested difficulty, the nearest supported level is used.
Mixed settings and weights are saved locally with the Random Pack controls.
Existing fixed Easy / Standard / Challenge Random Packs and manual per-engine difficulty settings remain unchanged.

No PDF renderer changes are required; PDFs consume the generated activities and therefore inherit the mixed difficulty allocation automatically.