M4.3: a broken build VM blocks its projects until it's ready

The driver can request_reimage(reason) when its build VM is broken past the
usual fixes (before the go-ahead too: it asks, it doesn't act). The harness
parks the work in progress (pushed to Gitea), then blocks the VM in the state
store (schema 5, vm_blocks): no item on any project on it is taken up, across
restarts. The operator hears once at top level, with the command to clear it,
and each blocked item's thread gets a line. `vm ready <project>` lifts the
block and trusts the VM's new host key; a reimaged VM is set up again by the
toolchain step, and the item that found it goes first.

loop.qnt gains the block (blocked_vm_runs_nothing) and a scenario; it found
that a reimage lost work in progress, hence parking first. The walk requests
reimages and recovers them, reimaged or not. The unreachable case reuses the
block in #137. Architecture P5 is softer, and D3 describes the block.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-dev/tests/test_state.py

@@ -121,3 +121,28 @@ def test_newer_schema_is_refused(db: Path) -> None:
         pass
     with pytest.raises(StateSchemaError, match="newer"):
         StateStore(db, schema_version=1)
+
+
+def test_a_vm_block_persists_across_reopen(db: Path) -> None:
+    with StateStore(db) as store:
+        store.block_vm("root@vm1", cause="broken", reason="pip crashes", work_item="x")
+        store.block_vm("root@vm1", cause="broken", reason="later", work_item="y")
+    with StateStore(db) as store:
+        [block] = store.vm_blocks().values()
+        assert (block.host, block.reason, block.work_item) == (
+            "root@vm1",
+            "pip crashes",  # the first block is kept
+            "x",
+        )
+        assert store.unblock_vm("root@vm1", cause="test") == block
+        assert store.unblock_vm("root@vm1", cause="test") is None
+        assert store.vm_blocks() == {}
+
+
+def test_a_version_4_store_gains_vm_blocks(db: Path) -> None:
+    with StateStore(db, schema_version=4) as store:
+        store._conn.execute("DROP TABLE vm_blocks")  # as a real version 4 store
+    with StateStore(db) as store:
+        assert store.schema_version == 5
+        store.block_vm("root@vm1", cause="broken", reason="r", work_item=None)
+        assert list(store.vm_blocks()) == ["root@vm1"]