fix: address review findings on the low-memory work

Nine CodeRabbit findings, five in code.

**Health state (the one that matters).** The non-dict guard did not cover a
dict missing fields the callers index directly, which is the shape actually
seen in the wild: a record carrying only circuit_state produced
`plugin clock-simple operation failed: 'circuit_state'` about fifty times a
minute with the panel frozen. The record is now completed against the
defaults per field rather than trusted or discarded wholesale. Per field
matters: a first pass rejected any incomplete record outright, which reset a
tripped breaker and real failure counts to healthy because one optional
field was absent -- an existing test caught it. Values of the wrong type
(a counter persisted as a string, an unknown circuit_state) fall back
individually, valid neighbours survive, and newer fields the schema has
grown since (degraded, degraded_reason) are carried through untouched.

**Cache ceiling.** MemoryCache.set() accepted entries without bound between
cleanup sweeps, which run every 300s by default, so a burst could take the
cache far past max_size -- the unbounded growth the limit exists to stop.
Eviction now runs under the same lock on every write, sharing one helper
with the periodic sweep so the two cannot drift.

**Installer, cgroups.** Only cgroup_enable=memory was checked, so a board
carrying that without cgroup_memory=1 reported success and got no change,
leaving MemoryMax= inert. Each parameter is now checked and appended
independently; verified against all four combinations, single line preserved.

**Installer, journald.** Persistence was inferred from /var/log/journal being
non-empty, which proves neither Storage=persistent nor a size cap -- the
directory survives a switch back to volatile. The effective configuration is
read instead (systemd-analyze cat-config, falling back to the conf files),
and an explicitly configured SystemMaxUse is preserved rather than
overwritten. Verified across volatile, persistent-without-cap,
persistent-with-user-cap, cap-without-storage, and commented-only configs.

**Dependency extras.** _extras_are_satisfied stopped at one level, so a
gated dependency that itself requests an extra (requests[socks]) passed on
the base distribution's version while the extra's own dependency was
missing, and pip was skipped. It now recurses, with a visited
(distribution, extras) set so a cycle terminates.

Docs: both kernel command-line paths documented (the installer falls back to
/boot/cmdline.txt), daemon-reload and restart added after the systemd
override example, memory exhaustion added to the SSH summary with its
power-cycle-only recovery, and a language on the fenced block for MD040.

Tests: five for the health-state repair including the exact wild shape and
that record_failure/record_success no longer raise against it, and one for
the cache ceiling. Both mutation-checked. Full suite 2927 passed, with the
one pre-existing tmpfs failure that also fails on main.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

test/test_plugin_health.py

@@ -91,3 +91,77 @@ def test_force_reload_refreshes_stale_in_memory_snapshot():
 
     # and it asked the cache to bypass the in-memory tier (memory_ttl=0).
     assert any(c.kwargs.get("memory_ttl") == 0 for c in cache.get.call_args_list)
+
+
+# --- persisted state that does not match the current schema -------------------
+#
+# A record on disk can be missing fields the callers index directly: a partial
+# write, a restored backup, or a state written by an older schema. Returning it
+# verbatim raises KeyError inside record_success / record_failure, which takes
+# the display down in a restart loop that survives reboots, because the bad
+# entry is on disk and gets read again on the way back up. Observed in the wild
+# as `plugin clock-simple operation failed: 'circuit_state'`, repeating ~50x a
+# minute with the panel frozen.
+
+_INDEXED_FIELDS = (
+    "consecutive_failures", "total_failures", "total_successes",
+    "last_success_time", "last_failure_time", "circuit_state",
+    "circuit_opened_time", "half_open_start_time", "last_error",
+)
+
+
+def _tracker_reading(persisted):
+    cache = _cache()
+    cache.get.return_value = persisted
+    return PluginHealthTracker(cache)
+
+
+def test_partial_state_is_completed_not_returned_raw():
+    """The shape seen in the wild: one field, everything else absent."""
+    state = _tracker_reading({"circuit_state": "closed"}).get_health_state("p")
+    for field in _INDEXED_FIELDS:
+        assert field in state, f"{field} missing; callers index it directly"
+
+
+def test_repair_keeps_real_failure_history():
+    """A record with genuine counts must not be reset to healthy just because
+    an optional field is absent -- that would clear a tripped breaker."""
+    state = _tracker_reading({
+        "consecutive_failures": 5,
+        "total_failures": 5,
+        "circuit_state": "open",
+    }).get_health_state("p")
+    assert state["consecutive_failures"] == 5
+    assert state["total_failures"] == 5
+    assert state["circuit_state"] == "open"
+
+
+def test_wrong_types_fall_back_per_field():
+    """A counter persisted as a string would pass a membership check and then
+    fail on the first += 1; an unknown circuit_state would take a branch the
+    breaker has no handling for."""
+    state = _tracker_reading({
+        "consecutive_failures": "3",
+        "circuit_state": "melted",
+        "total_failures": 7,
+    }).get_health_state("p")
+    assert state["consecutive_failures"] == 0
+    assert state["circuit_state"] == CircuitState.CLOSED.value
+    assert state["total_failures"] == 7, "valid neighbours must survive"
+
+
+def test_newer_fields_are_carried_through():
+    """degraded/degraded_reason are read with .get() and are not part of the
+    indexed set; repairing must not drop them."""
+    state = _tracker_reading({
+        "circuit_state": "closed", "degraded": True, "degraded_reason": "x",
+    }).get_health_state("p")
+    assert state["degraded"] is True
+    assert state["degraded_reason"] == "x"
+
+
+def test_recording_against_a_repaired_state_does_not_raise():
+    """The actual failure: record_failure indexing a field that was not there."""
+    tracker = _tracker_reading({"circuit_state": "closed"})
+    tracker.record_failure("p", Exception("boom"))
+    tracker.record_success("p")