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>

crates/punktfunk-core/src/lib.rs

@@ -38,6 +38,7 @@ pub mod input;
 pub mod packet;
 #[cfg(feature = "quic")]
 pub mod quic;
+pub mod reanchor;
 pub mod session;
 pub mod stats;
 pub mod transport;
@@ -61,7 +62,10 @@ pub use stats::Stats;
 /// TTL of a v2 envelope; `punktfunk_connection_next_rumble` is unchanged and drops it). Additive —
 /// the wire is backward-compatible (the envelope is a length-tolerant tail on 0xCA), so
 /// [`WIRE_VERSION`] is unchanged.
-pub const ABI_VERSION: u32 = 5;
+/// v6: added the `punktfunk_reanchor_gate_*` surface (post-loss freeze-until-reanchor gate for the
+/// Swift client; Rust embedders use [`reanchor::ReanchorGate`] directly). Additive, client-local —
+/// no wire change, so [`WIRE_VERSION`] is unchanged.
+pub const ABI_VERSION: u32 = 6;
 
 /// The punktfunk/1 **wire** version — what `Hello`/`Welcome` carry and hosts equality-check.
 /// Deliberately its own constant: [`ABI_VERSION`] tracks the embeddable **C surface**