geenforce: gate publishing on the library version, not on the manifest's own

The publish gate exists because a minor version bump that NARROWS behaviour is
not backward compatible: _CmmVersion arrived in lib 2.6, and an older lib does
not know the field, so every gated entry looks unfiltered and it installs every
PC-DMIS version it cannot detect, on every CMM, in one cycle.

It was comparing the fleet's reported library versions against manifestversion.
That is the manifest's own 'Version' field. For a share-imported manifest the
two numbering schemes happen to coincide; for a scope authored in code they do
not, and seed_display_scope writes '2.0' - which every kiosk exceeds. So the
gate passed on the scope that most needed it.

A scope now declares minlibversion. Unset, the requirement is DERIVED from what
the manifest actually uses, so a scope written before this column existed is
still judged on its contents rather than on a number that says nothing about the
library. Only features that narrow behaviour belong in that table; an addition
an old lib ignores harmlessly needs no floor. manifestversion remains the last
fallback, which preserves what share-imported manifests already relied on.

plugins/geenforce/migrations/versions/0004_scope_minlibversion.py

@@ -0,0 +1,49 @@
+"""A scope declares the client library it needs, separately from its own version.
+
+The publish gate exists to refuse a manifest the fleet's GE-Enforce library
+cannot read correctly. It was comparing reported library versions against
+`manifestversion` - the manifest's own 'Version' field, which for a share
+imported manifest happens to share the library's numbering but for a
+code-authored scope is whatever the author chose (seed_display_scope writes
+'2.0'). Every kiosk runs something newer than that, so the gate passed on the
+one scope most in need of it.
+
+`minlibversion` says the thing the gate actually wants to know. NULL means
+derive it from what the manifest uses, so existing scopes need no edit.
+
+Revision ID: geenforce0004minlib
+Revises: geenforce0003subtype
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = 'geenforce0004minlib'
+down_revision = 'geenforce0003subtype'
+branch_labels = None
+depends_on = None
+
+TABLE = 'manifestscopes'
+
+
+def _columns(bind):
+    insp = sa.inspect(bind)
+    if TABLE not in insp.get_table_names():
+        return None
+    return {c['name'] for c in insp.get_columns(TABLE)}
+
+
+def upgrade():
+    # Guarded like geenforce0003subtype and network0003prefix: on a fresh
+    # database the table is built from the models, which already declare this.
+    columns = _columns(op.get_bind())
+    if columns is None or 'minlibversion' in columns:
+        return
+    op.add_column(TABLE, sa.Column('minlibversion', sa.String(length=16),
+                                   nullable=True))
+
+
+def downgrade():
+    columns = _columns(op.get_bind())
+    if columns is None or 'minlibversion' not in columns:
+        return
+    op.drop_column(TABLE, 'minlibversion')