fix(apple): the drift test tripped Swift's static exclusivity check

CI caught what my local harness could not: reading `huge.count` inside the closure that already
holds `huge` exclusively is an exclusivity violation, so PunktfunkKitTests failed to compile.

The blind spot is worth recording. I verified `AudioRing` by compiling it against a standalone
harness whose bodies were TOP-LEVEL code, where Swift applies DYNAMIC exclusivity — the same
statement in a function body gets the static check and is a hard error. A harness that does not
share the shape of the thing it stands in for can be green for a reason the real build does not
have. The harness now puts every body in a method and compiles with
`-enforce-exclusivity=checked`.

Length now comes off the buffer pointer (`$0.count`), which is what the closure already owns.

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

clients/apple/Tests/PunktfunkKitTests/AudioRingDriftTests.swift

@@ -79,9 +79,11 @@ final class AudioRingDriftTests: XCTestCase {
         scratch.withUnsafeMutableBufferPointer { ring.read(into: $0.baseAddress!, count: want) }
         XCTAssertTrue(scratch.contains { $0 != 0 }, "should be playing after priming")
 
-        // Drain it dry with one oversized read, then feed a normal quantum again.
+        // Drain it dry with one oversized read, then feed a normal quantum again. The length comes
+        // off the buffer pointer, not off `huge`: touching the array inside the closure that is
+        // already holding it exclusively is an exclusivity violation.
         var huge = [Float](repeating: 0, count: 200 * perMS)
-        huge.withUnsafeMutableBufferPointer { ring.read(into: $0.baseAddress!, count: huge.count) }
+        huge.withUnsafeMutableBufferPointer { ring.read(into: $0.baseAddress!, count: $0.count) }
         let feed = [Float](repeating: 0.5, count: want)
         feed.withUnsafeBufferPointer { ring.write($0.baseAddress!, count: want) }
         scratch.withUnsafeMutableBufferPointer { ring.read(into: $0.baseAddress!, count: want) }