gates: the today-override announces itself; malformed no longer swallowed

Part 6 of the hub-blindness task, separable and done rather than dropped.

Both due_checks_gate.py and instructions_gate.py read FELHOM_GATE_TODAY so
their suites can control "today", and neither said so. A shell that still has
it exported -- exactly what a session doing gate-test work leaves behind --
made both gates evaluate against a fabricated date and pass in SILENCE. That
is this project's own named failure class: an instrument that can quietly
return the wrong answer is not a measurement. The seam is legitimate and
stays; the silence was the defect.

Both now print a loud line naming the variable, its value, and that the real
date is being ignored, before any verdict.

And instructions_gate.py no longer swallows a MALFORMED override: it used to
fall through to the real date without a word while due_checks_gate.py already
exited 2 on the same input -- one variable, two gates, disagreeing about what
a mistake means. Both exit 2 now.

Tests extended in both suites (42 and 73 assertions, green). Red-proof: the
announcement was deleted from due_checks_gate.py and its two assertions were
seen failing, then reverted.

Also adds REPORT-hub-blindness.md (topic-suffixed; the shared REPORT.md is
left alone per the parallel-session rule).

scripts/instructions_gate.py

@@ -121,12 +121,29 @@ def effective(text):
 
 
 def today_tuple():
-    """Local date as (y, m, d). Injectable via FELHOM_GATE_TODAY for the test suite."""
+    """Local date as (y, m, d). Injectable via FELHOM_GATE_TODAY for the test suite.
+
+    THE OVERRIDE ANNOUNCES ITSELF (R-339 session, 2026-08-18). It is a legitimate test seam and the
+    suite depends on it — but a shell that still has it exported, which is exactly what a session
+    doing gate-test work leaves behind, silently made every date comparison here evaluate against a
+    fabricated day and pass. That is this project's own named failure class: an instrument that can
+    quietly return the wrong answer is not a measurement. So the override is kept and made LOUD.
+
+    A MALFORMED override is now exit 2, not a silent fall-through to the real date. Quietly ignoring
+    it means the run the operator thought they were doing is not the run that happened — and it also
+    made this gate disagree with due_checks_gate.py, which already exited 2. Same seam, same rule.
+    """
     override = os.environ.get("FELHOM_GATE_TODAY")
     if override:
         m = ISO_DATE_RE.match(override.strip())
         if m:
+            print("!! FELHOM_GATE_TODAY=%s IS SET — this gate is evaluating against that date, NOT "
+                  "today's real date. Unset it for a real run." % override.strip())
             return tuple(int(g) for g in m.groups())
+        print("FAIL: FELHOM_GATE_TODAY=%r is set but is not YYYY-MM-DD." % override)
+        print("      Refusing to fall back to the real date: the run you think you are doing would")
+        print("      not be the run that happens. Fix or unset the variable.")
+        sys.exit(2)
     import datetime
 
     d = datetime.date.today()