feat(gems): add multiplayer gem game with custom content packs

- implement FastAPI and HTMX lobby and game interfaces
- support up to four human and AI-controlled players
- add configurable rules, victory conditions, and expansion modules
- support validated JSON cards, patrons, objectives, and outposts

python/gems/content.py

@@ -0,0 +1,55 @@
+"""Content-pack parsing, normalization, and JSON Schema publication."""
+
+from __future__ import annotations
+
+import hashlib
+import json
+from dataclasses import dataclass
+from typing import Any
+
+from pydantic import ValidationError
+
+from .domain.models import ContentPack
+
+MAX_PACK_BYTES = 512 * 1024
+
+
+@dataclass(frozen=True, slots=True)
+class ParsedPack:
+    """Validated content plus its canonical representation."""
+
+    pack: ContentPack
+    canonical_json: str
+    digest: str
+
+
+class ContentPackError(ValueError):
+    """A safe, user-visible content validation error."""
+
+
+def parse_content_pack(raw: bytes | str) -> ParsedPack:
+    """Validate a content pack and return stable canonical JSON."""
+    data = raw.encode() if isinstance(raw, str) else raw
+    if len(data) > MAX_PACK_BYTES:
+        error = "Content pack exceeds the 512 KiB limit"
+        raise ContentPackError(error)
+    try:
+        decoded: Any = json.loads(data)
+    except (UnicodeDecodeError, json.JSONDecodeError) as exc:
+        error = f"Invalid JSON: {exc}"
+        raise ContentPackError(error) from exc
+    try:
+        pack = ContentPack.model_validate(decoded)
+    except ValidationError as exc:
+        messages = []
+        for issue in exc.errors(include_url=False):
+            path = ".".join(str(part) for part in issue["loc"])
+            messages.append(f"{path or '$'}: {issue['msg']}")
+        raise ContentPackError("\n".join(messages)) from exc
+    canonical = json.dumps(pack.model_dump(mode="json", by_alias=False), sort_keys=True, separators=(",", ":"))
+    return ParsedPack(pack=pack, canonical_json=canonical, digest=hashlib.sha256(canonical.encode()).hexdigest())
+
+
+def content_pack_schema() -> dict[str, Any]:
+    """Return the authoritative version-one JSON Schema."""
+    return ContentPack.model_json_schema(by_alias=False)