feat: M4 stage 1 — the SwiftUI client is real: compiles, tested, first light on glass

The clients/apple scaffold is now a working macOS client, validated live against this
repo's host across the LAN: gamescope virtual output → NVENC HEVC → lumen/1 (GF(2¹⁶) FEC +
AES-GCM over UDP, QUIC control) → VideoToolbox → AVSampleBufferDisplayLayer at 720p60,
mouse/keyboard flowing back as QUIC datagrams into the host's gamescope EIS injector
(~3.7k events injected in one session).

LumenKit:
- LumenConnection: the predicted cbindgen compile fixes (C17 header spells the typedefs as
  integers while the enum constants import as a distinct Swift type — bridge by rawValue);
  close() is now safe from any thread (a close flag + pumpLock held across the blocking
  poll enforce the C contract "never close with a next_au in flight"; flag prevents
  lock-starvation by back-to-back polls).
- StreamView: per-pump cancellation token (reconnects can't double-pump), flush + re-gate
  on the next in-band parameter sets when the layer fails, no stale enqueue after restart.
- InputCapture: fractional-delta accumulation (sub-pixel motion isn't truncated away),
  pressed-state tracking with release-all on focus loss and stop() (nothing sticks down
  host-side), global-singleton ownership guard (GC has one handler slot per process),
  X1/X2 buttons, horizontal scroll, full keypad/CapsLock/ISO-102nd/PrintScreen/Menu VKs.
- LumenClient app shell (swift run LumenClient): connect form, fps/Mb-s HUD,
  LUMEN_AUTOCONNECT/LUMEN_MODE for scripted first-light runs.
- Tests: Annex-B byte-level units; real-codec round trip (VTCompressionSession-encoded
  HEVC rebuilt as the host's wire shape → AnnexB → VTDecompressionSession → pixels);
  test-loopback.sh (Swift client vs a real local m3-host over loopback — the Swift twin of
  c_abi_connection_roundtrip); RemoteFirstLightTests (full pipeline over the LAN).

Host/build fixes that fell out:
- The workspace builds on non-Linux again: gamestream audio (opus) and sendmmsg batching
  are now platform-gated with stubs/fallback, per the crate's "compiles everywhere" rule.
- Horizontal scroll was inverted end-to-end: the injectors negated BOTH axes onto the
  ei/wl axes, but GameStream's horizontal convention is positive = right
  (moonlight-qt/Sunshine pass it through unnegated) — only vertical flips now. This also
  un-inverts real Moonlight clients.
- AnnexB drops all zeros preceding a start code (trailing_zero_8bits padding), ffmpeg's
  policy, instead of leaking them into the preceding NAL.
- build-xcframework.sh: deployment targets pinned to the package floor + an otool guard —
  cargo does not fingerprint MACOSX_DEPLOYMENT_TARGET, so warm caches can silently ship
  too-new minos objects.

Adversarially reviewed (5-dimension multi-agent pass, every finding refutation-verified):
14 confirmed findings, all fixed above; the send-while-polling core-contract gap flagged
here is closed by the lumen/1 session-planes work (&self pulls + per-plane borrow slots).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Sources/LumenKit/AnnexB.swift

@@ -12,6 +12,9 @@ import Foundation
 
 public enum AnnexB {
     /// Split an Annex-B stream into NAL units (start codes 00 00 01 / 00 00 00 01 stripped).
+    /// All zeros immediately preceding a start code are dropped: they're either the
+    /// 4-byte-code prefix or `trailing_zero_8bits` padding, never NAL payload (emulation
+    /// prevention keeps 00 00 0x out of conforming NAL bytes) — same policy as ffmpeg.
     public static func nalUnits(in data: Data) -> [Data] {
         var nals: [Data] = []
         let bytes = [UInt8](data)
@@ -19,8 +22,11 @@ public enum AnnexB {
         var start = -1
         while i + 2 < bytes.count {
             if bytes[i] == 0, bytes[i + 1] == 0, bytes[i + 2] == 1 {
-                let codeStart = (i > 0 && bytes[i - 1] == 0) ? i - 1 : i
-                if start >= 0 {
+                var codeStart = i
+                while codeStart > 0, bytes[codeStart - 1] == 0 {
+                    codeStart -= 1
+                }
+                if start >= 0, start < codeStart {
                     nals.append(Data(bytes[start..<codeStart]))
                 }
                 start = i + 3