Functionally smoke-test each native in CI (load + open a PDF)

[Frooodle]

Problem

The natives are never functionally tested in CI. Per-platform jobs in snapshot.yml / publish-github-packages.yml build the real bridge and then only check the output dir is non-empty (test -n "$(ls -A native/dist/...)"); ci.yml builds a stub and compile-checks. The real integration tests are gated behind -Djpdfium.integration=true and never run in CI. So a native that builds but can't load (missing dep, bad libc, ABI break) ships green.

Change

Functionally smoke-test each freshly-built native through the production load path:

• NativeSmokeTest - loads the native via NativeLoader and opens a real PDF (PdfDocument.open → pageCount() >= 1). Gated on -Djpdfium.smoke=true so it only runs where a matching native is present (not in the stub PR build).
• nativeSmokeTest gradle task + a platform-selectable test native: testRuntimeOnly now honors -Pjpdfium.testNatives=<platform> (default linux-x64), so each CI job loads the native it just built instead of always linux-x64.
• snapshot.yml + publish-github-packages.yml: after bundling, install jextract and run :jpdfium:nativeSmokeTest -Pjpdfium.testNatives=<platform>. Covers linux-x64, linux-arm64, darwin-arm64, windows-x64. darwin-x64 is skipped (cross-compiled on an arm64 host, so the x64 native can't execute there) and keeps the file-exists check.

Validation

Ran the new task locally on a real host (Windows x64), staging the published windows native into native/dist/windows-x64/:

> Task :jpdfium:nativeSmokeTest
tests="1" skipped="0" failures="0" errors="0"
loadsNativeAndOpensPdf  PASSED (0.89s)

So the production load path + PDF open is exercised, not just "a file exists". The other platforms run the same task on their own native runner in CI.

Note: this is independent of the musl PR (#16); once both land, musl gets the same smoke for free via -Pjpdfium.testNatives=linux-musl-*.