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>

src/scribe/services/coverage.py

@@ -660,6 +660,11 @@ class ArchiveScan(NamedTuple):
 
     definitions: list[ArchiveShape]
     references: dict[str, dict[str, int]]   # path → class token → count
+    # EVERY file's repo-relative path, scannable or not (milestone 463). The
+    # platform markers are files this scan otherwise skips — go.mod,
+    # AndroidManifest.xml, a Dockerfile — so detection reads the names here
+    # rather than re-walking the tarball.
+    paths: tuple[str, ...] = ()
 
 
 def definitions_from_archive(blob: bytes) -> list[ArchiveShape]:
@@ -678,11 +683,14 @@ def scan_archive(blob: bytes) -> ArchiveScan:
     """
     shapes: list[ArchiveShape] = []
     references: dict[str, dict[str, int]] = {}
+    paths: list[str] = []
     with tarfile.open(fileobj=io.BytesIO(blob), mode="r:gz") as tar:
         for member in tar:
             if not member.isfile() or "/" not in member.name:
                 continue
             path = member.name.split("/", 1)[1]
+            if path:
+                paths.append(path)
             if not path or not scannable(path) or member.size > _MAX_FILE_BYTES:
                 continue
             handle = tar.extractfile(member)
@@ -708,7 +716,7 @@ def scan_archive(blob: bytes) -> ArchiveScan:
             )
             if refs:
                 references[path] = refs
-    return ArchiveScan(shapes, references)
+    return ArchiveScan(shapes, references, tuple(paths))
 
 
 # --- matching shapes against recorded locations ------------------------------
@@ -806,6 +814,8 @@ async def compute_coverage(
         return None
 
     served: list[tuple[str, str]] = []
+    # Every file path across the project's repos, for platform detection.
+    tree_paths: list[str] = []
     recorded = await _recorded_locations(user_id, project_id)
     # The proposer's canon catalog, read once per refresh and shared across
     # the project's repos (#2792).
@@ -822,6 +832,7 @@ async def compute_coverage(
         ref = binding.ref or await forge.default_branch(api_repo)
         scan = scan_archive(await forge.archive(api_repo, ref))
         definitions = scan.definitions
+        tree_paths.extend(scan.paths)
         # The head commit is provenance sugar on the ledger rows; failing to
         # learn it must not fail the sync — the ref names the point well
         # enough and the row timestamps carry the when.
@@ -857,6 +868,16 @@ async def compute_coverage(
     if not served:
         return None
 
+    # Platform detection (milestone 463) rides the same walk: the paths are in
+    # hand once. It only ever ADDS membership the project has no answer for,
+    # and it must not be able to fail the refresh it rides on.
+    try:
+        from scribe.services import platforms as platforms_svc
+
+        await platforms_svc.detect_for_project(project_id, tree_paths)
+    except Exception:
+        logger.warning("platform detection failed for project %s", project_id, exc_info=True)
+
     await shape_ledger.mark_canonicals(project_id, recorded)
     try:
         await shape_ledger.apply_derive_groups(project_id)