fix(forms): renaming a nested field checks the full name it would take

Renaming a field changes only its own /T, so MASTER.a renamed to b
becomes MASTER.b. set_props compared the bare new partial name with the
other fields' fully qualified names: renaming MASTER.a to b succeeded
while MASTER.b existed, leaving two fields with one fully qualified
name, which are one field with a shared value (ISO 32000-2 §12.7.4.2);
and renaming MASTER.a to c was refused because of an unrelated
top-level field c.

The check now compares the new full name, leaving the field itself out
so a kid keeping its own partial name still succeeds, and the error
names that full name. Top-level renames are checked as before.

parity/acrobat-features.toml

@@ -5040,7 +5040,7 @@ milestone = "M6"
 status = "shipped"
 commands = ["form.prepare"]
 tools = ["form_set_props"]
-evidence = ["crates/forms/src/tests.rs::locked_fields_only_take_unlocking", "crates/ui-egui/tests/prepare.rs::preview_fills_the_form_and_locked_fields_keep_their_properties"]
+evidence = ["crates/forms/src/tests.rs::locked_fields_only_take_unlocking", "crates/ui-egui/tests/prepare.rs::preview_fills_the_form_and_locked_fields_keep_their_properties", "crates/forms/src/tests.rs::renaming_a_kid_is_checked_against_the_full_name_it_takes", "crates/forms/src/tests.rs::renaming_a_top_level_field_is_still_checked_against_top_level_names"]
 notes = "rename in Field Properties ▸ General; Locked sets the widgets' annotation Locked flag (/F bit 8) and greys out the properties until unlocked; values can still be filled in"
 
 [[feature]]