Add wallpaper and icon-layout commands

[aluedeke]

Summary

Adds four CLI commands for inspecting and modifying iOS home-screen state on supervised devices, reverse-engineered from Apple Configurator's cfgutil set-wallpaper and the com.apple.springboardservices RPC.

• ios set-wallpaper <image> [--screen=lock|home|both] --p12file=... [--password=...] — sets the device wallpaper. Sends the MDM Settings command with item Wallpaper over the post-Escalate com.apple.mobile.MCInstall channel, exactly matching cfgutil's wire format (verified by DTrace).
• ios get-wallpaper [--output=wallpaper.png] — pulls the home-screen wallpaper PNG via springboardservices. No supervision required.
• ios get-icon-layout [--output=…] / ios set-icon-layout <file> — round-trippable JSON capture/restore of the home-screen layout (getIconState / setIconState).

The supervised auth handshake (Escalate / EscalateResponse / ProceedWithKeybagMigration) reuses the existing mcinstall.EscalateWithCertAndKey path; no new crypto is added.

Behavior caveats (documented in CLI help and Go doc-comments)

• iOS 16+ unified Lock + Home wallpaper: the device applies the image to both screens regardless of the Where field. Apple's own cfgutil exhibits the same behavior. The --screen flag is preserved for older iOS / forward-compat.
• get-wallpaper may EOF on iOS 18: known iOS-side regression also seen in pymobiledevice3 (doronz88/pymobiledevice3#1450). Code is correct.
• set-icon-layout constraint: iOS requires every installed app to occupy a slot. Per cfgutil docs: "unexpected behavior may occur if the given layout does not contain every icon on the device". Missing apps are re-paginated, not hidden.
• Edit-Pages hidden-page bit (iOS 14+) is not exposed by springboardservices, so a fetched layout will not include hidden pages, and pushing a layout cannot reproduce a hidden page.

Test plan

• go build ./... clean (only my touched packages; existing untracked ios/resign/ WIP excluded)
• go vet ./ios/mcinstall ./ios/springboard ./ clean
• set-wallpaper end-to-end on a real supervised iPhone SE (iOS 18.7.1) — wallpaper changes verified visually
• --screen=lock|home|both all return "ok"; behavior matches cfgutil exactly (both screens get set on iOS 16+, unavoidable)
• get-icon-layout → set-icon-layout round-trip on real device returns "ok", layout preserved
• Negative tests: invalid --screen, missing --p12file, P12_PASSWORD env-var fallback, all behave correctly
• get-wallpaper on iOS < 18 (no test device available)

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