fix: look up internal tags in freshTag deduplication

`Coe String Tag` made `tags.contains attempt` search for `Tag.provided`,
so colliding auto-generated `Tag.internal` names were never detected.
Fixes #897.

Author: Chessing234

src/verso-manual/VersoManual/Basic.lean

@@ -417,7 +417,9 @@ def freshTag [Monad m] [MonadStateOf TraverseState m] (hint : String) (id : Inte
   let mut numPart : Option Nat := none
   repeat
     let attempt := tagStr strPart numPart
-    if (← get).tags.contains  attempt then
+    -- Lookup must use `Tag.internal`: `attempt` is a `String`, and the `Coe String Tag`
+    -- instance would otherwise search for `Tag.provided`, so generated tags never collide.
+    if (← get).tags.contains (Tag.internal attempt) then
       numPart := some <| numPart.map (· + 1) |>.getD 0
     else break
   let tag := tagStr strPart numPart