import: remediate review findings — ledger gate, docs, message split

Delegates the refuse-gate to ledgerDegraded(), lifts its console message into a
pure testable fold, fixes stale doc line citations and an inaccurate outcome-enum
comment, and splits the rename counter into collision-vs-sanitize.

tests/test_import_landing.cpp

@@ -372,7 +372,9 @@ static void testReimportingIntoTheSourceProjectLandsBesideAnUntouchedOriginal()
     CHECK(third.landing.outcome == ImportOutcome::Landed);
     CHECK(book.bank("bank-b3")->displayName == "B 3");
     CHECK(*book.index("bank-b") == originalB);
-    // Six distinct files: the original two plus two per re-import, never overwritten.
+    // Four distinct files: two per re-import, never overwritten. Bank "B"'s own two
+    // entries were seeded index-only above (book.index("bank-b")->add), never written
+    // to disk, so they don't add to this count.
     CHECK(scratch.bankFiles().size() == 4);
 }