refactor(data): restructure content directory for galaxy scale

Replace flat content/districts/ with hierarchical campaign/system/
station/district structure. Path mirrors canonical IDs, enables
glob-based discovery, and is multi-campaign/DLC ready.

- git mv 46 files preserving history
- New metadata: campaign.yaml, system.yaml, station.yaml
- Rewrite content.yaml with glob-based district discovery
- Add campaign, system, station JSON schemas
- Update district schema: hierarchy fields derived from path
- Update npc-profile schema: accept district-scoped IDs
- Add TODO to 17 dialogue/monologue pool files for generator revision
- Update _meta/README.md with new hierarchy documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

content/_schema/system.schema.json

@@ -0,0 +1,26 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "system.schema.json",
+  "title": "Star System Metadata",
+  "description": "System definition file — one per system directory (D-036).",
+  "type": "object",
+  "required": ["display_name"],
+  "additionalProperties": false,
+  "properties": {
+    "display_name": {
+      "type": "string",
+      "minLength": 1
+    },
+    "description": {
+      "type": "string"
+    },
+    "stations": {
+      "type": "array",
+      "items": {
+        "type": "string",
+        "pattern": "^[a-z][a-z0-9-]*$"
+      },
+      "uniqueItems": true
+    }
+  }
+}