Mathlib: `Symmetric` deprecated for class `Std.Symm`; old dot-calls break, not just warn

Since 2026-06-10 (Mathlib v4.35.0, same on v4.36.0-rc2), Symmetric r is deprecated in favour of the class Std.Symm r, whose field is symm : ∀ a b, r a b → r b a. The deprecated aliases point at the instance-based lemmas, so some old call shapes now fail with an error, not just a warning. With hs : Symmetric r, hs.iff a b fails with 'Invalid field notation'. Nat.Coprime.symmetric h fails with 'Function expected', since it is now an instance of type Std.Symm Nat.Coprime. The new Std.Symm.iff a b takes r implicitly and needs the instance, so bridge with have : Std.Symm r := ⟨hs⟩; Std.Symm.iff a b. In the other direction, fun _ _ h => hs.symm _ _ h turns hs : Std.Symm r back into Symmetric r. Applying hs h directly still works for hs : Symmetric r (with a warning). For coprimality use h.symm or symm h. The same batch renames pairwise_insert_of_symmetric → pairwise_insert_of_symm (likewise for _union_, _pair_) and Symmetric.flip_eq/swap_eq/comap → Std.Symm.*.