feat(runtime): report native memory held by wrappers to V8, forward memory pressure

[edusperoni]

What

The runtime never told V8 about native memory kept alive by JS wrappers. V8 only weighs its own heap (plus ArrayBuffer backing stores, which it counts itself) when scheduling GCs. So a wrapper holding a multi-MB UIImage looks like ~50 bytes and can sit uncollected until iOS jetsams the app. This PR adds reporting at three levels of confidence and forwards OS memory pressure.

Accounting primitive

• ExternalMemoryCharge (wrapping v8::ExternalMemoryAccounter, the non-deprecated replacement for AdjustAmountOfExternalAllocatedMemory) hangs off BaseDataWrapper. Whichever path deletes the wrapper (finalizer, __releaseNativeCounterpart, teardown, an adapter's -dealloc on another thread) returns the charge.
• Release is thread-safe: V8's decrease is an atomic counter update that V8 itself calls from GC worker threads. The isolate's gate is pinned for the call. Once the gate is closed the decrease is skipped, because disposal drops the isolate's total anyway.

Tier 1: exact sizes the runtime allocates

• interop.alloc(n) charges n bytes to the adopted pointer.

Tier 2: estimated sizes for objects JS likely owns

• Estimators: NSData → length; UIImage / CGImage → bytesPerRow * height; CVPixelBuffer → CVPixelBufferGetDataSize. They are resolved once per class and cached in Caches.
• CoreGraphics and CoreVideo functions are resolved with dlsym, so the runtime gains no link dependencies.
• NSDataAdapter is excluded, since V8 already counts its JS buffer.
• Symbol images are skipped, since -CGImage rasterizes them.
• Applied only when JS most likely holds the last reference:
  • new X(...)
  • initializers
  • +1 returns (copy / new / Create)
  • class factory methods
• Instance getters (view.image) are not charged, because the receiver usually keeps the object alive and the GCs triggered by charging it could not free anything.

Tier 3: library-declared sizes

• interop.setExternalSize(obj, bytes) replaces the charge on a native object, pointer, reference, struct, block or function reference; 0 clears it. interop.getExternalSize(obj) reads it back.
• Class, protocol and type objects throw TypeError, because they live as long as the isolate and the charge could never be returned.
• Byte counts must be in [0, 2^34], otherwise RangeError. V8 aborts the process, rather than throwing, on a single adjustment above its sanity limit.
• This is the hook for @nativescript/core ImageSource, canvas and plugins, which know their real footprint.

Memory pressure

• A process-wide DISPATCH_SOURCE_TYPE_MEMORYPRESSURE source maps normal/warn/critical to MemoryPressureLevel::kNone/kModerate/kCritical and calls Isolate::MemoryPressureNotification on every live isolate.
• The notification runs outside the isolate registry lock, with each isolate's gate pinned. Off-thread, V8 requests a GC interrupt and posts a task to the isolate's runner.

Known limitations

• Estimates are taken once, at wrap time; later NSMutableData growth is not tracked.
• A decoded-bitmap estimate for an image loaded from a file overstates its memory until it is drawn.
• interop.bufferFromData on a charged NSData counts the bytes twice: once for the NSData wrapper, once for the ArrayBuffer.
• interop.setExternalSize / getExternalSize typings belong in @nativescript/types and are a follow-up.

Tests

TestRunner/app/tests/ExternalMemoryTests.js (12 specs) covers:

• setting, replacing and clearing charges
• rejecting invalid targets and byte counts
• release through GC and explicit release
• interop.alloc
• the factory, constructor, +1 and getter paths
• CGImage/UIImage bitmap estimates

Full suite on a dedicated simulator: 1794 specs, 0 failures. The memory-pressure path has no automated test because the simulator has no reliable way to raise a memory-pressure event.

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