feat(family): platforms declared at inception and detected from bound repos (milestone 463 step 2, #4988)

A project's platforms decide which family ideas reach it. This step makes membership answerable from every door:

- services/platforms.py: the global catalog (writes are admin-only and duplicate-gated by slug); pure marker detection; and membership reads and writes. Detection only ADDS, and only where nobody has answered. It never overrides a declared or rejected row and never removes one.
- coverage: the archive scan now carries every path, and the refresh runs detection fail-open.
- inception: a platforms choice (slugs, or null for unanswered). The list is the whole answer: members left out of it become rejected.
- MCP: list_platforms and set_project_platforms; enter_project and get_project carry the project's platforms.
- REST: /api/platforms (admin writes) and /api/projects/<id>/platforms.
- UI: a platforms checklist on the inception card, a Family tab on ProjectView, and a Platforms admin tab in Settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/helpers.py

@@ -512,3 +512,18 @@ def skill_text(name: str) -> str:
     folder = pathlib.Path(__file__).resolve().parents[1] / "plugin" / "skills" / name
     refs = sorted(p for p in folder.glob("*.md") if p.name != "SKILL.md")
     return "\n\n".join(p.read_text() for p in [folder / "SKILL.md", *refs])
+
+
+def forge_tarball(files: dict[str, bytes], top: str = "widget") -> bytes:
+    """A gzipped tar shaped like a forge archive: every file under one
+    top-level directory (repo-ref/), which the scan strips."""
+    import io
+    import tarfile
+
+    buf = io.BytesIO()
+    with tarfile.open(fileobj=buf, mode="w:gz") as tar:
+        for path, data in files.items():
+            info = tarfile.TarInfo(f"{top}/{path}")
+            info.size = len(data)
+            tar.addfile(info, io.BytesIO(data))
+    return buf.getvalue()