[civilwarinfrance]

Fixes #2989. A UIBoxLayout whose children all have visible=None raises ValueError while finding their minimum size. With mixed children, it also reserves gaps for collapsed widgets and leaves unused space when allocating elastic children.

Count only participating children in both sizing calculations, handle zero participants as an empty box, and invalidate the cached minimum when child visibility changes. Padding/borders and collapsed child rectangles are preserved. visible=False keeps its existing layout participation.

Added 19 regression cases covering both orientations, empty/all-collapsed boxes, before/between/after positions, actual child geometry, padding/borders, hidden controls, dynamic changes and the real UIManager lifecycle. Original code fails 13 cases; the fixed cases all pass.

Validation on macOS ARM64/Python 3.14 with the lockfile's pytest/Ruff/Mypy versions and pinned Pyright:

• Full unit suite: 1641 passed, 9 failed. Unchanged development: 1622 passed, the same 9 failed, covering existing manager-sizing, text-rendering and HiDPI tests. No full-suite pass claimed.
• Full Ruff lint, import sorting and formatting checks pass.
• Mypy passes all 400 source files; Pyright reports 0 errors/warnings.
• Whitespace check passes. Other-platform and hosted checks remain unverified.

AI assistance was used to identify, reproduce, implement and test this fix. The issue was opened before code changes, following the contribution guide.