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.

src/shell/package/import_landing.cpp

@@ -3,6 +3,7 @@
 
 #include "shell/package/import_landing.h"
 
+#include <cassert>
 #include <cstdint>
 #include <filesystem>
 #include <system_error>
@@ -118,12 +119,23 @@ ImportLanding landPackage(const std::string& packageAbsPath,
 
 bool applyImportedBank(BankBook& book, const std::string& bankId,
                        const package::ImportPlan& plan, const RecordBirth& recordBirth) {
+    // An empty std::function throws std::bad_function_call on invoke; every real caller
+    // supplies one, so an empty one here is a caller bug, not a runtime condition to
+    // recover from — enforce the contract rather than let it surface as an uncaught
+    // exception out of an extension action.
+    assert(recordBirth && "applyImportedBank: RecordBirth must not be empty");
     if (!book.createBank(bankId, plan.bankDisplayName)) return false;
 
     BankModel* index = book.index(bankId);
     for (const package::PlannedEntry& e : plan.entries) {
         if (e.action != EntryAction::Land) continue;
-        index->add(e.sample);
+        const AddResult added = index->add(e.sample);
+        // planImport already deduped Land entries by hash against an empty destination
+        // bank (this same freshly-created one), so a Collapsed add here would mean the
+        // plan and the book disagree — that would silently undercount reportSuccess's
+        // landedCount rather than fail loudly.
+        assert(added == AddResult::Added && "planImport's Land entries must not collapse");
+        (void)added;
         // Unconditional on the add's outcome: the file exists either way, and an
         // unrecorded file is permanently unreclaimable.
         recordBirth(e.sample);