feat: .kc Layer 1 — manifest auto-creation and platform file associations

kc_format.py:
- Auto-create silo/manifest.json with default fields (UUID, timestamps,
  username) when saving a .kc file that lacks one
- Update modified_at timestamp on each save
- KC_VERSION = 1.0

Platform integration:
- kindred-create.desktop: add application/x-kindred-create MIME type
- kindred-create.xml: register .kc glob patterns with dedicated MIME type
  (application/x-kindred-create), separate from .fcstd type

resources/kindred-create.desktop

@@ -7,7 +7,7 @@ Icon=kindred-create
 Terminal=false
 Type=Application
 Categories=Graphics;Science;Engineering;
-MimeType=application/x-extension-fcstd;x-scheme-handler/kindred;
+MimeType=application/x-extension-fcstd;application/x-kindred-create;x-scheme-handler/kindred;
 Keywords=CAD;3D;modeling;engineering;design;parametric;
 StartupNotify=true
 StartupWMClass=KindredCreate

resources/kindred-create.xml

@@ -1,11 +1,18 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
   <mime-type type="application/x-extension-fcstd">
-    <comment>Kindred Create Document</comment>
-    <comment xml:lang="en">Kindred Create Document</comment>
+    <comment>FreeCAD Document</comment>
+    <comment xml:lang="en">FreeCAD Document</comment>
     <icon name="kindred-create"/>
     <glob pattern="*.FCStd"/>
     <glob pattern="*.fcstd"/>
     <glob pattern="*.FCSTD"/>
   </mime-type>
+  <mime-type type="application/x-kindred-create">
+    <comment>Kindred Create Document</comment>
+    <comment xml:lang="en">Kindred Create Document</comment>
+    <icon name="kindred-create"/>
+    <glob pattern="*.kc"/>
+    <glob pattern="*.KC"/>
+  </mime-type>
 </mime-info>

src/Mod/Create/kc_format.py

@@ -0,0 +1,102 @@
+"""
+kc_format.py — .kc file format support.
+
+Handles two responsibilities:
+1. Round-trip preservation: caches silo/ ZIP entries before FreeCAD's C++
+   save rewrites the ZIP from scratch, then re-injects them after save.
+2. Manifest auto-creation: ensures every .kc file has silo/manifest.json.
+"""
+
+import json
+import os
+import uuid
+import zipfile
+from datetime import datetime, timezone
+
+import FreeCAD
+
+# Cache: filepath -> {entry_name: bytes}
+_silo_cache = {}
+
+KC_VERSION = "1.0"
+
+
+def _default_manifest():
+    """Generate a default silo/manifest.json for new .kc files."""
+    now = datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")
+    username = os.environ.get("USER", os.environ.get("USERNAME", "unknown"))
+    return {
+        "kc_version": KC_VERSION,
+        "silo_instance": None,
+        "part_uuid": str(uuid.uuid4()),
+        "revision_hash": None,
+        "created_at": now,
+        "modified_at": now,
+        "created_by": username,
+    }
+
+
+class _KcFormatObserver:
+    """Document observer that preserves silo/ entries across saves."""
+
+    def slotStartSaveDocument(self, doc, filename):
+        """Before save: cache silo/ entries from the existing file."""
+        if not filename.lower().endswith(".kc"):
+            return
+        if not os.path.isfile(filename):
+            return
+        try:
+            with zipfile.ZipFile(filename, "r") as zf:
+                entries = {}
+                for name in zf.namelist():
+                    if name.startswith("silo/"):
+                        entries[name] = zf.read(name)
+                if entries:
+                    _silo_cache[filename] = entries
+        except Exception:
+            pass
+
+    def slotFinishSaveDocument(self, doc, filename):
+        """After save: re-inject cached silo/ entries and ensure manifest."""
+        if not filename.lower().endswith(".kc"):
+            _silo_cache.pop(filename, None)
+            return
+        entries = _silo_cache.pop(filename, None)
+        try:
+            with zipfile.ZipFile(filename, "a") as zf:
+                existing = set(zf.namelist())
+                # Re-inject cached silo/ entries
+                if entries:
+                    for name, data in entries.items():
+                        if name not in existing:
+                            zf.writestr(name, data)
+                            existing.add(name)
+                # Ensure silo/manifest.json exists
+                if "silo/manifest.json" not in existing:
+                    manifest = _default_manifest()
+                    zf.writestr(
+                        "silo/manifest.json",
+                        json.dumps(manifest, indent=2) + "\n",
+                    )
+                else:
+                    # Update modified_at timestamp
+                    raw = zf.read("silo/manifest.json")
+                    manifest = json.loads(raw)
+                    now = datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")
+                    if manifest.get("modified_at") != now:
+                        manifest["modified_at"] = now
+                        # ZipFile append mode can't overwrite; write new entry
+                        # (last duplicate wins in most ZIP readers)
+                        zf.writestr(
+                            "silo/manifest.json",
+                            json.dumps(manifest, indent=2) + "\n",
+                        )
+        except Exception as e:
+            FreeCAD.Console.PrintWarning(
+                f"kc_format: failed to update .kc silo/ entries: {e}\n"
+            )
+
+
+def register():
+    """Connect to application-level save signals."""
+    FreeCAD.addDocumentObserver(_KcFormatObserver())