plans: installer I4 hostcheck row brief, split out of I3 (darkwing row 233 G2, sage-1650)

docs/plans/2026-10-20_installer-rows.md

@@ -227,3 +227,36 @@ host prep into the command is #437.
 
 Filbert approves the manifest. The test file, the cli suite and every `scripts/test-*.sh` are green on
 Sage's gate, with the mutants killed. Target: in review 10-24, landed 10-25 (sage-1528 line 12).
+
+## I4 hostcheck: the bwrap profile check, pins and boot unit
+
+Sage, 2026-10-21, split out of I3 (darkwing row 233 G2, sage-1650). Owner Darkwing, reviewer Filbert, gate
+owner Sage. Darkwing builds it right after I3, and it must land before the ops-01 proof.
+
+### Problem
+
+The host run sheet (I3, line 182 above) lists r6's `hostcheck` among the host items for any host that runs
+bwrap, and I3's A1 refuses `profile-drift` unless `/usr/local/lib/mosaic-stack/hostcheck.sh --check` exits 0.
+No hostcheck exists on refactor. It's r6 B2 (row 195 DESIGN.md at b7239bb66, :262-303 and :839-860), sized at
+150-200 lines of bash and 350-450 of test. That's work separate from the upgrade sequence: the root-side bwrap
+stop and a network fetch.
+
+### What ships
+
+r6 B2 as designed:
+- `case1-pins.sh` holds the pins: the profile sha256, the apparmor-profiles version and the bubblewrap pin.
+- `hostcheck.sh` covers the seven items in three modes:
+  - `--check` exits 0 or 1.
+  - `--enforce` stops bwrap: `apparmor_parser -R`, then `root:root 0700` on `/usr/bin/bwrap`. It writes
+    `/etc/mosaic-stack/DRIFT` and a `hostcheck.log` line, and exits 0.
+  - `--upstream` compares the current apparmor-profiles deb to the pin.
+- `mosaic-hostcheck.service`, a boot oneshot.
+- `case1-hostcheck.test.mjs` uses fakes for every host tool and covers r6's seven case groups and seven
+  mutants.
+
+The I3 run sheet's hostcheck item points at this row. I3's A1 and its fake don't change.
+
+### Gate
+
+Filbert approves the manifest. The test file, the cli suite and every `scripts/test-*.sh` are green on
+Sage's gate, with the mutants killed. Target: in review 10-25, landed 10-26, before the ops-01 proof.