specs: design for dropping Cgo on mobile (iOS purego, Android non-goal)

[changkun]

Design spec only — no implementation — following the #118 precedent for the Linux design PR.

Summary

Completes the Cgo-removal track (darwin #117, Linux #120, BSD #121) for the two remaining mobile backends, after investigating both. The investigation produced a clean split on a single test: can the platform's clipboard backend function without importing a package that itself requires Cgo?

• iOS — actionable. clipboard_ios.go imports no x/mobile package (UIPasteboard is process-global), and GOOS=ios CGO_ENABLED=0 go build std passes. Port it to the same purego/objc pasteboard binding shipped for darwin (darwin: remove the Cgo / C-toolchain dependency (purego) (#69) #117), retargeted to UIPasteboard; delete clipboard_ios.m. Simpler than darwin: text-only, no image, no TIFF, no Dlsym.
• Android — non-goal, documented. The backend needs x/mobile/app.RunOnJVM → mobileinit.RunOnJVM → C.lockJNI (Cgo), and gomobile Android apps are -buildmode=c-shared via the NDK regardless. No source change; the spec records the full dependency analysis and rejects the purego-over-JNI alternative (churn, no toolchain win).

Notable findings

• Silent-degradation bug (the one darwin: remove the CGO / C toolchain dependency #69 fixed on darwin) also affects both mobile targets today: under CGO_ENABLED=0, clipboard.go resolves to undefined: initialize/read/write/watch — neither a backend nor the graceful stubs link. The iOS port fixes it for iOS.
• Latent use-after-free in clipboard_ios.m: return (char *)[str UTF8String] returns a pointer into autoreleased storage without copying. The purego port copies bytes out, with a regression test.
• Benefit is scoped honestly: this removes Cgo from the library, not from a gomobile app (x/mobile's iOS scaffolding is itself Cgo). The spec spells out the library-vs-app distinction so the claim isn't overstated.

No open tracking issue exists for mobile (closed #8/#70); references the sibling #69/#25 and builds on #117.