Untag FR-PLAT-AND-1 from the two places that record its absence

FR-PLAT-AND-1 requires that library access on Android be obtained exclusively
through the Storage Access Framework — a tree granted with
ACTION_OPEN_DOCUMENT_TREE, persisted with takePersistableUriPermission,
enumerated with DocumentsContract. None of those three appears anywhere.

What carried the tag was a type and a negation.

`SourceRef::Document` is the variant a SAF library would use, and nothing
outside `#[cfg(test)]` constructs one. `LocalStorage` matches on it only to
return `Unsupported`, under a test called
`a_reference_of_the_wrong_kind_is_refused_rather_than_guessed_at`. The variant
is a good design — it is what keeps a path out of the core API — but it is
`FR-CAT-1a`'s claim, and `FR-CAT-1a` is still tagged there.

`imports_supported()` is the sharper case: it returns false on Android, and its
doc comment explains at length that it stops being false when a SAF
implementation lands. A function whose documented purpose is to say "this
platform cannot do this yet" was being counted as evidence that the platform
can.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

platform/dr-plat/src/volumes.rs

@@ -59,7 +59,7 @@ impl Volume {
     }
 }
 
-/// TRACES: FR-CAT-10 | FR-PLAT-AND-1 | NFR-PORT-1
+/// TRACES: FR-CAT-10 | NFR-PORT-1
 /// Whether an import can reach a card on this platform at all.
 ///
 /// **Not the same question as "did [`volumes`] find anything".** An empty list