feat(client): freeze-until-reanchor loss recovery on Android + Apple via shared core gate

After unrecoverable loss the host keeps sending delta frames that reference a
picture the client never received; hardware decoders conceal these as gray/
garbage with a success status. Linux already withheld them and held the last
good frame until a proven clean re-anchor — this brings that behavior to the
Android and Apple clients.

Extract the Linux pump's freeze state machine into a shared `ReanchorGate` in
punktfunk-core (reanchor.rs, 18 tests) exposed over the C ABI (ABI v6, additive —
no wire change) for the Swift clients. Migrate the Linux/Deck pump
(pf-client-core) onto it as the parity proof (no-op refactor). Then wire:

- Android (decode.rs, both sync + async loops): arm on the frame-index gap, a
  pts-keyed flag map carries the wire flags to the output-buffer release, fold
  the gate per drained output, gate.poll replaces the dropped-climb block.
- Apple Stage2Pipeline (default): arm on a gap (new noteFrameIndexGap), withhold
  at the ring-submit seam (CAMetalLayer holds its last drawable), poll
  framesDropped, fold VT decode errors through the no-output streak.
- Apple StreamPump (stage-1): fold at enqueue, withhold via
  kCMSampleAttachmentKey_DoNotDisplay so the layer keeps decoding (reference
  chain intact) but holds the last displayed frame.
- Apple VideoDecoder: thread the AU's wire flags to the async decode callback via
  a retained FrameContext refcon (replaces the receivedNs bit-pattern scalar).

Lifts only on a proven re-anchor (IDR / RFI anchor / 2nd recovery mark) with a
500 ms backstop so a lost re-anchor can never freeze forever. Apple: swift build
clean, 123/123 tests pass (incl. VideoToolboxRoundTripTests). On-glass
loss-injection validation still owed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkKit/Connection/PunktfunkConnection.swift

@@ -450,6 +450,21 @@ public final class PunktfunkConnection {
         _ = punktfunk_connection_note_frame_index(h, frameIndex, nil)
     }
 
+    /// Like `noteFrameIndex`, but also reports whether the core saw a FORWARD frame-index gap — the
+    /// signal that intervening frames were lost and the following AUs reference a picture that never
+    /// arrived. The post-loss re-anchor gate arms its display freeze on a gap (the earliest, most
+    /// precise loss trigger — ahead of the `framesDropped` climb). Same core side effect as
+    /// `noteFrameIndex` (the throttled RFI request); call it for every received AU. Returns false
+    /// after close.
+    public func noteFrameIndexGap(_ frameIndex: UInt32) -> Bool {
+        abiLock.lock()
+        defer { abiLock.unlock() }
+        guard let h = handle, !closeRequested else { return false }
+        var gap = false
+        _ = punktfunk_connection_note_frame_index(h, frameIndex, &gap)
+        return gap
+    }
+
     /// Cumulative access units the host→client reassembler dropped as unrecoverable (FEC couldn't
     /// rebuild them). The video pump polls this and calls `requestKeyframe()` when it climbs — the
     /// correct loss trigger under the host's infinite GOP, where unrecoverable loss yields