refactor: eliminate 4 as any casts in extraction.ts

- Added 'contain' field to FastExtractedElement.style interface, removing
  (s as any).contain property smuggling (line 682)
- Replaced (assertion as any) duck-typing in getSelectorsFromAssertion
  with instanceof FluentRelation/FluentQuantifier/FluentAssertion checks
- Replaced (assertion as any) duck-typing in buildFluentLabel with
  same instanceof pattern
- Structural baseline ratcheted: asAny 101→98, nullishZero 62→61

scripts/.structural-baseline.json

@@ -1,5 +1,5 @@
 {
-  "asAny": 101,
+  "asAny": 98,
   "emptyCatch": 17,
-  "nullishZero": 62
+  "nullishZero": 61
 }