Houdini 20.5: harden primitive user data rates

[rolledhand]

This supports OpenMoonRay/openmoonray#228.

Summary:

• Skips primitive user data with impossible rate/count matches instead of truncating to a guessed rate.
• Emits a targeted warning when malformed primitive attributes are skipped.
• Updates the package Python baseline away from Python 3.9 for the H20.5/Python 3.11 build path.

Validation:

• Covered by the H20.5 hdMoonray primitive-user-data and fixture render path in Moonray macOS: update build compatibility for Houdini 21.0.680 openmoonray#228.