Spike (#33 phase 1): one drawn-checkbox heuristic shared by iOS and Android

[SlyWombat]

Phase 1 of #33, as that issue specifies it: prove the packaging and binding work on
a small operation that already has assertions on every platform, before the
text-edit layer depends on it. detectCheckboxSquares is that operation.

This is a spike, not release work. It is not meant to ride along with the 1.1
payload that just merged. What it produces is the evidence for ADR-002.

The exit criterion from #33

detectCheckboxSquares is deleted from the C#, Kotlin and Swift sources and
implemented once in the shared core, and the existing fixtures assert identical
rects on all three platforms — with the iOS and Android release pipelines green
end to end.

This PR does Android and iOS. Windows is deliberately left out, per the
migration order in #33: it is the only shipping product, it needs an MSVC/CMake
step added to its pipeline to produce megapdf_core.dll, and IPdfEngine should
be preserved as an adapter so no MegaPDF.App code changes. Tracked separately.

What landed

core/megapdf_core.{h,cpp} — the ABI and its one implementation. The rules that
let three toolchains agree:

• C only. No C++ types cross, no exceptions escape.
• Handles are void* — pass your FPDF_PAGE straight through. The header
  includes nothing from pdfium, so binding code needs only that one file.
• Caller-owned buffers, count-then-fill. Keeps JNI and P/Invoke marshalling
  trivial.
• Coordinates come back in crop space. Getting that wrong is iOS: search highlights and tap targets use MediaBox coordinates on cropped pages #30 exactly, so
  it happens once here instead of in each binding.

Android compiles it into the existing .so (one CMake line; no second binary to
ship, no runtime ABI boundary) and its JNI entry point becomes marshalling only —
which is what engine.cpp's own header comment always claimed it was. iOS compiles
it into the app target and reaches it through a bridging header. Both hand-written
copies of the heuristic are gone.

What CI decides

The existing fixture assertions are the check: CheckboxTest on Android and
CheckboxTests on iOS already assert exact rects from fixture.pdf, and they now
exercise the shared implementation instead of two separate ones. If they still pass
unchanged, the architecture is proven for the rest; if the packaging fights back,
we learned it for the price of one small function.

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