Clamp brush opacity after pressure modifiers

[Loggableim]

Summary

• clamp final brush opacity after node and pressure multipliers
• clamp the decal cursor preview through the same range
• add focused regression checks

Root cause

Brush opacity could exceed 1.0 after pressure and node multipliers, producing invalid paint and preview opacity.

Validation

• python paint/tests/test_issue_2084.py
• Local native project generation is currently blocked by the bundled amake stack-overflow error; GitHub platform builds will provide compile validation.

Fixes #2084