ViolationDiff treats a changed violation at the same index as not new

[JeroenDeDauw]

ViolationDiff identifies violations by (property, code, valuePartIndex) and ignores args, so replacing one offending value with a different offending value at the same position is classified as pre-existing rather than new.

Consequence: guards that rely on "only new violations block" do not fire on an in-place swap. This includes the always-blocking relation-target-unresolvable-source violation from #1265: an existing unresolvable relation target can be edited into a different unresolvable target without the write being blocked, even though writing that target fresh would be refused.

Fix direction: include args (or the offending value) in the violation identity, while keeping genuinely unchanged statements non-blocking.

_{AI-authored — Claude Code, Fable 5 (max); found during the #1265 code review, filed on Jeroen's behalf; issue text not human-reviewed.}