fix(skills): document that registered props are not ID properties on 5.0+

[TMHSDigital]

Closes #354.

Problem

On 4.x a registered bpy.props value was stored as an ID property. On 5.0+ it isn't, so the 4.x idioms break with a KeyError or ValueError, or silently miss the property. The custom-properties skill called its operations "version-stable" and never mentioned this.

Measured

Live run (--factory-startup), bpy.types.Scene.p = IntProperty(default=1); scene.p = 5. Binaries report 4.5.11 LTS, 5.1.2 and 5.2.1 LTS:

Access	4.5.11	5.1.2	5.2.1
scene.p	5	5	5
scene["p"]	5	KeyError	KeyError
scene.get("p")	5	None	None
"p" in scene.keys()	True	False	False
del scene["p"]	resets to 1	KeyError	KeyError
path_resolve('["p"]') (driver path)	5	ValueError	ValueError
path_resolve("p"), property_unset, is_property_set	ok	ok	ok
bl_system_properties_get	absent	present	present

A .blend saved by 4.5.11 (value 9) opens in 5.2.1 with scene.p == 9.

Change

• custom-properties:
  • a "Registered properties are not ID properties on 5.0+" section with the table;
  • the branch-free recipe: attribute access, property_unset, is_property_set, plain path;
  • a new common mistake fix: correct version-accuracy defects across skills and snippets #7;
  • a note that del id["registered"] is not a reset.
• drivers-and-app-handlers: data_path now distinguishes '["custom"]' (an ID property) from a registered prop's plain name.
• snippets/cross-version-property-delete.py: warns about the reverse case (32 lines).
• examples/cross-version-property-delete:
  • new check_registered(), run on every version: asserts which side of the split the prop sits on, that attribute access and plain paths work, and that property_unset resets it; exit 8;
  • new falsifier --subscript-registered, which reads scene["bdt_counter"] the 4.x way;
  • catalog row gets that second falsifier with min_version: "5.0";
  • README exit table, witness text and smoke line updated; gallery regenerated.

Evidence (live run, three versions)

Run	4.5.11	5.1.2	5.2.1
default	0 (in_keys=True split=False)	0 (in_keys=False split=True)	0 (same as 5.1.2)
--subscript-registered	0 (subscript still works; catalog records SKIP via min_version)	8 (KeyError … read it as scene.bdt_counter)	8
--skip-delete (existing)	7	7	7

• check_smoke_catalog, check_exit_code_readme, check_falsifier_targets, check_skill_refs and check_site_links pass.
• Not run: the --output render path. check_registered() touches only a temporary Scene property, which it unregisters in finally, and the still is driven by the lamps' keys(), which are unchanged.

🤖 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, 7 skipped, 0 failed;143 passed, 1 skipped, 0 failed;143 passed, 0 skipped, 0 failed;