fix: fall back to non-pmr BoundedPriorityQueue on runtimes without std::pmr

[Leppard]

Summary

BoundedPriorityQueue is backed by std::pmr (multiset + monotonic_buffer_resource + unsynchronized_pool_resource). On Apple platforms those runtime symbols only exist in the system libc++ from iOS 17.0 / macOS 14.0 / tvOS 17.0 / watchOS 10.0 (_LIBCPP_AVAILABILITY_HAS_PMR → _LIBCPP_INTRODUCED_IN_LLVM_16).

libc++ currently leaves the pmr availability markup empty (see the // TODO: Enable std::pmr markup once llvm/llvm-project#40340 ... note in <__configuration/availability.h>), so using std::pmr below those versions compiles without any diagnostic but aborts at dyld load time:

Symbol not found: std::__1::pmr::memory_resource::~memory_resource()

That crashes every app at launch on iOS < 17 — see #1071 (also confirmed there on production App Store builds and real devices).

Approach

Following @mdydek's preference in #1071 (keep std::pmr, add an STL fallback for older runtimes):

• Gate the std::pmr implementation behind a new AUDIOAPI_HAS_PMR macro, auto-detected from libc++'s own _LIBCPP_AVAILABILITY_HAS_PMR so it tracks each platform's threshold automatically (Android keeps pmr). It can be overridden by defining AUDIOAPI_HAS_PMR before including the header.
• When pmr is unavailable, back the same std::multiset with an in-object, non-pmr pool allocator (FixedBlockPool + PoolAllocator) that preserves the original design intent: a fixed buffer, node recycling via an intrusive free list, and no heap allocation — so the audio-thread / real-time behavior is kept on the fallback too, not regressed to a heap-backed std::multiset.

The container type and the entire public API are unchanged, so iOS 17+ / Android keep the existing std::pmr fast path untouched — only sub‑iOS‑17 (and equivalent) builds compile the fallback.

Tests

Adds common/cpp/test/src/utils/BoundedPriorityQueueTest.cpp. It #defines AUDIOAPI_HAS_PMR 0 so the fallback path is exercised even on CI hosts whose libc++ has pmr (the default path is already compiled into the rest of the suite). Covers:

• ascending order + capacity bounds (full → rejected),
• pop returns the smallest element first,
• node recycling under 100k insert/erase cycles (proves the free list reclaims; no pool exhaustion),
• heterogeneous lowerBound / upperBound (transparent comparator),
• extract → mutate key → reinsert with hint, and range erase.

Verified locally: builds clean for arm64-apple-ios15.1 with zero pmr symbols in the resulting object (nm), the default (pmr) branch still compiles on a pmr-capable host, and all 4 gtest cases pass. (ASan/UBSan/TSan variants left to CI — the local macOS 26 ASan runtime hangs at startup even for a trivial program.)

Notes

• No podspec change needed — the ios_min_version = '14.0' claim becomes correct again with the fallback in place.
• Happy to adjust the macro name/placement, or split the fallback allocator into its own header, if you'd prefer.

Fixes #1071