test(f6): host E2E — toggle, advance-on-accept, pin, persist, untracked (SLICE-4)

F6 §6.8. Drives the programmatic seam ingress (propose + accept, no LLM):
opened baseline → toggle opens cowriting-baseline diff → type (baseline steady)
→ accept advances past the landing (INV-18) → operator delta → pin → on-disk
baseline outside .threads (INV-19) → toggle closes (normal editor active) →
untracked warns.

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

test/e2e/fixtures/workspace/docs/diffview.md

@@ -0,0 +1,7 @@
+# Diff view fixture
+
+The baseline opening paragraph stays put so the diff suite can anchor on it.
+
+A target sentence Claude will rewrite via the seam.
+
+A closing paragraph for the operator to edit by hand.