Correct the invariant claimed in the charterBytes comment

The comment said CharterReceipt never pairs a null digest with a non-zero
byte count. It can. compose() derives the digest with digestOf(), which
returns null for blank text, while the byte count is getBytes().length,
which does not. A whitespace-only role charter on a profile with no MCP
produces exactly that pair.

No behaviour change. The gate already omits both fields on that path, which
is the right answer — a size with no digest would describe an artifact we
cannot fingerprint. Only the stated reason was wrong, and a false invariant
in a comment is worse than no comment, because the next reader will widen
the gate on the strength of it.

fleetd/src/main/java/dev/ltms/fleet/session/SessionManager.java

@@ -872,11 +872,17 @@ public final class SessionManager implements TurnListener {
         // records whether a role charter was configured ("fleet.charters.<role>") or only the reply
         // charter was composed ("none").
         // #604: charterBytes rides along with charterSha256, not with charterSource — it is only
-        // meaningful as the digest's companion (a length turns "they differ" into "by how much"),
-        // and the receipt only ever pairs a non-null digest with a real byte count (CharterReceipt's
-        // own contract: no composed charter is an explicit null digest AND a zero byte count, never
-        // one without the other). A member with no composed charter at all reports charterSource and
-        // nothing else, exactly as before this change.
+        // meaningful as the digest's companion (a length turns "they differ" into "by how much").
+        // A member with no composed charter reports charterSource and nothing else, as before.
+        //
+        // Gating on the DIGEST rather than on the receipt is deliberate, and the two are not
+        // always null together. CharterReceipt.compose() derives the digest with digestOf(), which
+        // returns null for BLANK text, while the byte count is composed.getBytes().length, which
+        // does not. So a whitespace-only charter (a blank fleet.charters.<role> on a profile with
+        // no MCP, so no reply charter is appended) yields a null digest beside a non-zero size.
+        // Reporting a size with no digest would say "they differ by N bytes" about an artifact we
+        // cannot fingerprint, so this gate omits both. Never widen it to the receipt-level null
+        // check without deciding what that case should report.
         if (session.charterReceipt() != null) {
             m.put("charterSource", session.charterReceipt().charterSource());
             if (session.charterReceipt().charterSha256() != null) {