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/test_pattern_coverage.py

@@ -8,11 +8,9 @@ deliberately reuse the definitions test_write_path_trigger stages for the
 hook; extending one detector means extending both, and this comment is the
 tripwire.
 """
-import io
 import json
 import shutil
 import subprocess
-import tarfile
 from pathlib import Path
 
 import pytest
@@ -29,7 +27,7 @@ from scribe.services.coverage import (
     shapes_from_archive,
 )
 from scribe.services.shape_ledger import location_covers
-from tests.helpers import ensure_user
+from tests.helpers import ensure_user, forge_tarball
 
 PLUGIN = Path(__file__).resolve().parents[1] / "plugin"
 
@@ -173,14 +171,7 @@ def test_scannable_gates_prose_vendored_and_sourcemaps():
 # --- unit: reading shapes out of a forge tarball -----------------------------
 
 
-def _tarball(files: dict[str, bytes], top: str = "widget") -> bytes:
-    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()
+_tarball = forge_tarball  # shared with tests/test_platforms.py
 
 
 TREE = {