Fix iOS build on Unity 6.5

[MaxHeimbrock]

Background

This fixes #359

Unity 6000.5 re-architected the iOS export: the engine library is no longer
Libraries/libiPhone-lib.a but a static Frameworks/UnityRuntime.framework
(which still bundles the old CELT/libvpx objects that conflict with the opus/vpx
inside liblivekit_ffi.a). Our iOS post-build protection was hardwired to
libiPhone-lib.a, so on 6000.5 it silently no-oped and apps crashed at runtime
in ec_tell_frac (confirmed on device with the Meet sample).

On top of that, the Meet and Agents samples' own BuildPostProcessor passed the
now-null file GUID for libiPhone-lib.a into RemoveFileFromBuild, failing every
iOS build on 6000.5 with an ArgumentNullException.

The exported frameworks build phase is also no longer labeled /* UnityFramework */;
build logs show that marker didn't even match on 6000.3, so the link-order rewrite
was already a no-op there and only the CELT strip was protecting older builds.

Changes

• IosLinkOrderDiagnostics now resolves the engine library per export layout:
  Libraries/libiPhone-lib.a (up to 6000.4) or Frameworks/UnityRuntime.framework
  (6000.5+). Reorder, CELT strip, and the diagnostic all operate on the resolved
  library; behavior on the old layout is unchanged.
• Locate the UnityFramework frameworks build phase by content (the phase linking
  liblivekit_ffi.a) instead of the /* UnityFramework */ label, which also
  repairs the reorder/diagnostic on pre-6.5 exports.
• Meet & Agents samples: guard the libiPhone-lib.a remove/re-add so iOS builds
  no longer fail on 6000.5.
• README: document both export layouts.

Verified on Unity 6000.5.5f1 (device): export logs show the link-order fix applied,
18 conflicting CELT objects stripped from UnityRuntime, diagnostic OK, and the
Meet app no longer crashes.

[xianshijing-lk]

lgtm in general, one question

[xianshijing-lk]

Don't we have script to auto fix the problem and make sure we load liblivekit_ffi.a before libiPhone-lib.a ?

[MaxHeimbrock]

Yes, the script works until Unity 6.4, but in 6.5 Unity apparently changed their lib composition and naming.

A customer reported the crash: #359
They also already confirmed the fix works for them as well. I will now test if it still works in Unity 2022 or Unity 6.3, but that behaviour should still be intact.

[MaxHeimbrock]

Confirmed the old way of building is not touched in lower Unity versions.

[xianshijing-lk]

lgtm, I just missed the BuildPostProcessor.cs. Please ignore my previous comment