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_integration_inception.py

@@ -37,14 +37,16 @@ async def seeded():
 async def test_decide_applies_every_effect_and_records_last(seeded):
     owner, pid = seeded["owner"], seeded["pid"]
     defaults = await inception_svc.current_defaults(owner, pid)
-    assert set(defaults) == {"design_system_id", "design_systems", "systems"}
+    assert set(defaults) == {
+        "design_system_id", "design_systems", "systems", "platforms", "project_platforms",
+    }
     assert defaults["systems"] == 0 and defaults["design_system_id"] is None
 
     out = await inception_svc.decide(owner, pid, via="mcp", choices={
         "design_system_id": None,
         "seed_systems": True,
     })
-    assert set(out["effects"]) == {"design_system_id", "systems_seeded"}
+    assert set(out["effects"]) == {"design_system_id", "systems_seeded", "platforms"}
     catalog = await canonical_svc.list_canonical_systems()
     assert len(out["effects"]["systems_seeded"]) == len(catalog)
     # Seeded Systems come out mapped, not needing a later reconciliation.
@@ -56,7 +58,9 @@ async def test_decide_applies_every_effect_and_records_last(seeded):
         project = await s.get(Project, pid)
     assert inception_svc.is_decided(project)
     assert project.inception["via"] == "mcp" and project.inception["decided_by"] == owner
-    assert project.inception["choices"] == {"design_system_id": None, "seed_systems": True}
+    assert project.inception["choices"] == {
+        "design_system_id": None, "seed_systems": True, "platforms": None,
+    }
     # Re-deciding with seed again mints nothing twice.
     again = await inception_svc.decide(owner, pid, via="ui", choices={"seed_systems": True})
     assert again["effects"]["systems_seeded"] == []