Add Trace.concat notation and apply at call sites

Introduce `tr₁ ++< he >++ tr₂` scoped notation for `Trace.concat`
(precedence 65, right-associative, mirroring `++`) and use it
throughout Properties.lean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lean/Spa/Language/Traces.lean

@@ -48,6 +48,8 @@ noncomputable def Trace.concat {g : Graph} {idx₁ idx₂ idx₃ idx₄ : g.Inde
   | single hbs => exact Trace.edge hbs he tr₂
   | edge hbs he' _ ih => exact Trace.edge hbs he' (ih he tr₂)
 
+scoped notation:65 tr₁:66 " ++< " he " >++ " tr₂:65 => Trace.concat tr₁ he tr₂
+
 /-- A beginning-to-end trace corresponding to the CFG `g`. -/
 inductive EndToEndTrace (g : Graph) (ρ₁ ρ₂ : Env) : Type
   | intro (idx₁ : g.Index) (idx₁_mem : idx₁ ∈ g.inputs)