purego: support structs on Android and iOS

[hajimehoshi]

What issue is this addressing?

Closes #466

What type of issue is this addressing?

feature

What this PR does | solves

Enables passing and returning C structs by value on Android and iOS for
amd64 and arm64, in both RegisterFunc/RegisterLibFunc calls and NewCallback
callbacks — matching the support already available on macOS, Linux, and Windows
(#236, #237).

Why it works

The struct-marshalling code was already compiled into the iOS/Android builds and
only gated off by runtime.GOOS string checks. Their ABIs are identical to
platforms that already work:

• iOS arm64 == macOS arm64 (Apple ABI); the iOS simulator (amd64) is System V.
• Android amd64 == System V; Android arm64 == standard AAPCS64.

So this routes GOOS=ios through the Darwin path via a new isDarwin helper,
treats GOOS=android like Linux, opens the ensureStructSupported gate to
ios/android, adds them to the getCallbackStruct callback switches, and extends
the Darwin ARM64 stack-argument reads in the callback path to iOS.

Tests / CI

Adds an Android emulator CI job (amd64 and arm64) that runs the test suite.
Because the emulator has no C toolchain, the test libraries are cross-compiled on
the host with the NDK and loaded via a new PUREGO_TEST_PREBUILT_LIBDIR env var
instead of being compiled at run time. getSystemLibrary also learns Android's
libc (and iOS, for a future job).

Reviewer notes

• I could not run the Android emulator job locally (no emulator/NDK in my
  environment), so it may need a CI iteration to settle image/arch specifics —
  notably the arm64-v8a image under full emulation on the x86_64 runner, and
  dlopen("libc.so") for the adb-shell user. The harness change itself is
  verified on the host: the full suite passes through PUREGO_TEST_PREBUILT_LIBDIR.
• An iOS Simulator job is left as a TODO in the workflow (lower marginal value,
  since the iOS arm64 ABI matches macOS arm64, which the macOS runner already
  exercises).

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Authored by Claude (Claude Code) on behalf of @hajimehoshi.

[Copilot]

Pull request overview

This PR extends PureGo’s struct-by-value ABI support to Android and iOS (amd64/arm64), aligning the runtime GOOS gating with ABIs already supported on macOS/Linux/Windows, and adds CI coverage to exercise the Android paths.

Changes:

• Allow struct arguments/returns on GOOS=android and GOOS=ios by widening ensureStructSupported, routing iOS through a new isDarwin() helper, and treating Android like Linux in callback struct decoding.
• Update callback stack-reading/struct marshalling logic to apply the Darwin ARM64 packing rules on iOS as well.
• Add an Android emulator workflow job and a test harness path for loading prebuilt shared libraries via PUREGO_TEST_PREBUILT_LIBDIR; teach tests to locate Android libc.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
syscall_unix.go	Uses isDarwin() to apply Darwin ARM64 stack unpacking on iOS as well as macOS.
struct_arm64.go	Generalizes Darwin-vs-Linux ARM64 struct handling to include iOS/Android; expands callback struct support switches.
struct_amd64.go	Expands callback struct support switches to include iOS/Android on amd64.
README.md	Updates platform notes to reflect struct-by-value support on Android/iOS amd64/arm64.
func.go	Opens struct support gate for android/ios and introduces isDarwin() helper used across ABI logic.
func_test.go	Adds Android/iOS system library resolution and supports prebuilt test shared libraries via env var.
.github/workflows/test.yml	Adds an Android emulator CI job (amd64/arm64) that runs the test binary with prebuilt libraries.

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[hajimehoshi]

@TotallyGamerJet Please take a look, thanks

[TotallyGamerJet]

LGTM