Take the read-back back out of the persist; a write verdict belongs only where evidence crosses the plugin boundary

Its false gated six undo points, so an unverified byte-equality assumption could have silently removed Ctrl-Z for a bank mutation that landed.

src/shell/persist/session.h

@@ -108,16 +108,14 @@ public:
     void bumpBankGeneration() { ++bankGeneration_; }
 
     // Serializes book/view/tail to ext state, clears the retired legacy
-    // `bank_index` key. No-ops with no active/saved project.
+    // `bank_index` key. No-ops with no active/saved project. Returns true iff
+    // a persist happened, so a caller can skip an undo block when nothing was written.
     //
-    // Returns true iff the `banks` key READ BACK as exactly what this call wrote —
-    // the only per-key observation available under a shared extname
-    // (wire::extStateWriteLanded owns why SetProjExtState's own return cannot
-    // answer it). A false therefore covers four things without distinguishing
-    // them: no active project, an unsaved project, a rejected write, and a
-    // read-back that could not complete. The sibling keys (view/tail/ledger/
-    // version/generation) are written but NOT verified, so no caller may read
-    // this as "everything persisted" — only as "the bank state is in the project".
+    // "Happened" is the writes being ISSUED, and a false means exactly two things —
+    // no active project, or an unsaved one — with NOTHING written under either. SIX
+    // callers turn a false straight into a discarded undo point, so this must never
+    // grow a third, OBSERVATIONAL failure mode: a false negative would silently
+    // remove the Ctrl-Z for a bank mutation that landed.
     bool saveToActiveProject();
 
     // Report-only prune dry-run: feeds the pure core with (present, referenced,
@@ -151,7 +149,9 @@ public:
     // Write the ingest assignment request (`assign_request` key): "the active
     // sampler instance should now play THIS sample." `wire` is pre-encoded
     // (assignment_request.h); a sibling one-shot write, not part of
-    // saveToActiveProject's blob. Returns true iff written.
+    // saveToActiveProject's blob. Same two-condition return as that call: false
+    // means no active/saved project and nothing was written, true means the write
+    // was ISSUED — never that the project took it.
     bool writeAssignmentRequest(const std::string& wire);
 
     // Detects a project load or Save-As and reacts. Driven by REAPER's