fix(skills): re-register driver functions on file load

[TMHSDigital]

Closes #387.

Problem

bpy.app.driver_namespace is reset on every file load. The skill and snippets/driver-with-custom-function.py registered the custom function only in register(). On the first file a user opens:

• the driver evaluates with the function missing;
• it raises NameError and is disabled (driver.is_valid = False);
• it stays disabled even after the function is re-added.

Change

• Snippet:
  • restore_driver_function(), a @persistent load_post handler, re-registers the function and resets is_valid on drivers on objects, shape keys and materials;
  • symmetric register()/unregister();
  • the header explains the reset.
• Skill:
  • the same pattern, plus a note to extend the ID collections;
  • the compatibility table row now says "reset on file load";
  • a new common-mistakes entry.

Evidence

Proven by live run (--factory-startup; binaries report 5.2.1 LTS and 4.5.11 LTS). The probe imports the real snippet, attaches its smoothstep driver, evaluates at frame 50, then does save_as_mainfile → open_mainfile → re-evaluates:

Mode	5.2.1 z after reopen / is_valid	4.5.11
New register() (with load_post handler)	2.4621 / True	2.4621 / True
Old pattern (register only)	0.0 / False, NameError	0.0 / False
Old pattern + function re-added after load, no is_valid reset	0.0 / False	0.0 / False

Before the save, z = 2.4621 in every mode. The last row shows why the is_valid reset is required, not just the re-registration.

By inspection only: the snippet is 61 lines (limit 75).

🤖 Generated with Claude Code

[TMHSDigital]

CI evidence before merge: all checks pass ( 13 pass ). Blender Smoke on PR head: Blender 5.2.2 LTS / Blender 4.5.14 LTS. Summaries (happy path; falsifiers) per leg: 141 passed, 3 skipped, 0 failed;136 passed, 6 skipped, 0 failed;143 passed, 1 skipped, 0 failed;142 passed, 0 skipped, 0 failed;