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/persist/session.h

@@ -98,6 +98,13 @@ public:
     // the records. That is not a hole in the pairing rule above: the rule exists so an
     // absent record is never read as a definite answer, and this exposes strictly less
     // than the pair. The package import gates on it before it opens a file picker.
+    //
+    // A tradeoff, not the only route: `pruneDryRun()` already exposes the same degraded
+    // pair via `PruneReport::ledgerUnreadable`/`ledgerFutureVersion`, with no new
+    // accessor needed. Rejected because that route is genuinely worse for a gate: it
+    // drags a full bank-folder enumeration and every live instance's FX scan onto a
+    // check that only needs to know "can I write a record", and it shapes an import
+    // decision as an answer borrowed from prune's report rather than the session's own.
     tracking::LedgerStatus ledgerStatus() const { return trackingStatus_; }
 
     // The version that last wrote the active project: PreVersioning (no