make so-yaml PG-canonical and add pillar-change reactor stack

Two coupled changes that together let so_pillar.* be the canonical
config store, with config edits driving service reloads automatically:

so-yaml PG-canonical mode
- Adds /opt/so/conf/so-yaml/mode (and SO_YAML_BACKEND env override) with
  three values: dual (legacy), postgres (PG-only for managed paths),
  disk (emergency rollback). Bootstrap files (secrets.sls, ca/init.sls,
  *.nodes.sls, top.sls, ...) stay disk-only regardless via the existing
  SkipPath allowlist in so_yaml_postgres.locate.
- loadYaml/writeYaml/purgeFile now route to so_pillar.* in postgres
  mode: replace/add/get all read+write the database with no disk file
  ever appearing. PG failure is fatal in postgres mode (no silent
  fallback); dual mode preserves the prior best-effort mirror.
- so_yaml_postgres gains read_yaml(path), is_pg_managed(path), and
  is_enabled() so so-yaml can answer "is this path PG-managed and is
  PG up" without reaching into private helpers.
- schema_pillar.sls writes /opt/so/conf/so-yaml/mode = postgres after
  the importer succeeds, so flipping postgres:so_pillar:enabled flips
  so-yaml's behavior in lockstep with the schema being live.

pg_notify-driven change fan-out
- 008_change_notify.sql adds so_pillar.change_queue + an AFTER trigger
  on pillar_entry that enqueues the locator and pg_notifies
  'so_pillar_change'. Queue is drained at-least-once so engine restarts
  don't lose events; pg_notify is just the wakeup signal.
- New salt-master engine pg_notify_pillar.py LISTENs on the channel,
  drains the queue with FOR UPDATE SKIP LOCKED, debounces bursts, and
  fires 'so/pillar/changed' events grouped by (scope, role, minion).
- Reactor so_pillar_changed.sls catches the tag and dispatches to
  orch.so_pillar_reload, which carries a DISPATCH map of pillar-path
  prefix -> (state sls, role grain set) so adding a new service to
  the auto-reload list is a one-line edit instead of a new reactor.
- Engine + reactor wiring is gated on the same postgres:so_pillar:enabled
  flag as the schema and ext_pillar config so the whole stack flips
  on/off together.

Tests: 21 new cases (112 total, all passing) covering mode resolution,
PG-managed detection, and PG-canonical read/write/purge routing with
the PG client stubbed.

salt/postgres/schema_pillar.sls

@@ -54,6 +54,10 @@ so_pillar_postgres_wait_ready:
 {%   do sql_files.append('007_drift_pgcron.sql') %}
 {% endif %}
 
+# 008 always applies — pg_notify-driven change fan-out is what the salt-master
+# pg_notify_pillar engine consumes. Without it reactor wiring sees no events.
+{% do sql_files.append('008_change_notify.sql') %}
+
 {% for sql_file in sql_files %}
 so_pillar_apply_{{ sql_file | replace('.', '_') }}:
   cmd.run:
@@ -87,7 +91,7 @@ so_pillar_master_key_configure:
           exit 1
         fi
     - require:
-      - cmd: so_pillar_apply_006_rls_sql
+      - cmd: so_pillar_apply_{{ sql_files[-1] | replace('.', '_') }}
 
 # Run the importer once after the schema is in place. Idempotent — re-runs
 # with no SLS edits produce zero row changes.
@@ -97,6 +101,29 @@ so_pillar_initial_import:
     - require:
       - cmd: so_pillar_master_key_configure
 
+# Flip so-yaml from dual-write to PG-canonical for managed paths now that
+# the schema and importer are both in place. Bootstrap files (secrets.sls,
+# postgres/auth.sls, ca/init.sls, *.nodes.sls, top.sls, ...) remain on disk
+# regardless because so_yaml_postgres.locate() raises SkipPath for them.
+so_pillar_so_yaml_mode_dir:
+  file.directory:
+    - name: /opt/so/conf/so-yaml
+    - user: socore
+    - group: socore
+    - mode: '0755'
+    - makedirs: True
+
+so_pillar_so_yaml_mode_postgres:
+  file.managed:
+    - name: /opt/so/conf/so-yaml/mode
+    - contents: postgres
+    - user: socore
+    - group: socore
+    - mode: '0644'
+    - require:
+      - file: so_pillar_so_yaml_mode_dir
+      - cmd: so_pillar_initial_import
+
 {% else %}
 
 so_pillar_disabled_noop: