docs(encryption): Stage 6E-2c — fix stale comment on MissingSidecar test

Claude r6 caught a stale comment that predated the codex P1 r2
narrowing of the nil-return branch. The MissingSidecar test was
written when refuseStartupOnActiveRaftCutover silently returned
nil on any read error; the fail-closed fix narrowed that to
encryption.IsNotExist only. The test itself was already correct
(it asserts the missing-file case returns nil) — only the
docblock contradicted the actual contract.

Rewrote the comment to point at the BadSidecarPropagates sibling
test as the canonical reference for the non-NotExist branch.

Author: bootjp

main_encryption_write_wiring_test.go

@@ -214,10 +214,14 @@ func TestRefuseStartupOnActiveRaftCutover_NoSidecarPath(t *testing.T) {
 }
 
 // TestRefuseStartupOnActiveRaftCutover_MissingSidecar pins the
-// nil-on-missing-file behaviour: the Stage 6C-1 startup guard
-// is the authoritative refuser of malformed sidecars; this helper
-// only cares about a present sidecar with a non-zero cutover
-// index, and silently returns nil on any ReadSidecar error.
+// nil-on-missing-file behaviour: a missing sidecar is the
+// pre-bootstrap state and there is no raft envelope cutover to
+// refuse on. Only encryption.IsNotExist errors return nil here;
+// non-NotExist read errors (malformed JSON, schema violation)
+// propagate as a refusal — see the sibling
+// TestRefuseStartupOnActiveRaftCutover_BadSidecarPropagates that
+// pins that branch (codex P1 round-1 r2 narrowed the nil-return
+// from "any read error" to IsNotExist only).
 func TestRefuseStartupOnActiveRaftCutover_MissingSidecar(t *testing.T) {
 	t.Parallel()
 	if err := refuseStartupOnActiveRaftCutover(t.TempDir() + "/absent.json"); err != nil {