instructions_gate: CLOSED-ITEMS.md is the third register source

Compressing a finished row out of OPEN-ITEMS.md into CLOSED-ITEMS.md is the
documented housekeeping act. The gate read only OPEN-ITEMS.md and ROADMAP.md, so
the first compression (ef6ac6f) turned every citation of a compressed item into
'a reference to nothing' and failed CI on the next push, in felhom-controller,
for a rule file nobody had touched.

CLOSED-ITEMS.md now answers 'does this ID exist', and answers 'closed' for the
rows it owns. OPEN-ITEMS.md stays the sole authority on OPENNESS: a row it
already claims as open is not overridden.

Both controls still convict: an ID present nowhere fails, and a citation claiming
a closed item is still open fails.

scripts/instructions_gate.py

@@ -482,6 +482,12 @@ def register_state(workspace_root):
     base = os.path.join(workspace_root, "felhom.eu", "documentation", "backlog")
     reg = os.path.join(base, "OPEN-ITEMS.md")
     hist = os.path.join(base, "ROADMAP.md")
+    # CLOSED-ITEMS.md is the third source, and it is not optional. Compressing a finished row out of
+    # OPEN-ITEMS.md into it is the DOCUMENTED housekeeping act — and on 2026-08-22 (commit ef6ac6f)
+    # the first such compression turned every citation of a compressed item into "a reference to
+    # nothing", failing CI on the next push for a file nobody had touched. The register did not
+    # break; the gate's idea of where the register lives did.
+    closed = os.path.join(base, "CLOSED-ITEMS.md")
     if not os.path.exists(reg):
         return None
 
@@ -491,6 +497,10 @@ def register_state(workspace_root):
     if os.path.exists(hist):
         with io_open(hist) as fh:
             hist_text = fh.read()
+    closed_text = ""
+    if os.path.exists(closed):
+        with io_open(closed) as fh:
+            closed_text = fh.read()
 
     state = {}
     for line in reg_text.split("\n"):
@@ -518,9 +528,17 @@ def register_state(workspace_root):
                 state[m.group(1)] = "closed"
             break
 
+    # A row that OWNS a line in CLOSED-ITEMS.md is CLOSED — but only if OPEN-ITEMS.md did not
+    # already claim it as open work. OPEN-ITEMS.md stays the authority on openness; this file only
+    # answers "does this ID exist at all", and answers "closed" for the ones it holds.
+    for line in closed_text.split("\n"):
+        m = re.match(r"\|\s*\*\*(R-\d+[a-z]?)\*\*\s*\|", line)
+        if m and m.group(1) not in state:
+            state[m.group(1)] = "closed"
+
     mentioned = set(re.findall(r"\bR-\d+[a-z]?\b", reg_text)) | set(
         re.findall(r"\bR-\d+[a-z]?\b", hist_text)
-    )
+    ) | set(re.findall(r"\bR-\d+[a-z]?\b", closed_text))
     for item in mentioned:
         state.setdefault(item, "historical")
     return state