fix(ios): build one simulator arch for --remote and key iOS simulator builds by arch

[janicduplessis]

Description

stim ios --remote proxy|eas builds with -destination 'generic/platform=iOS Simulator' because no remote UDID exists at build time (#1212 boots the remote device after the build). With a generic destination Xcode forces ONLY_ACTIVE_ARCH=NO and compiles every ARCHS value, so a remote build compiled arm64 and x86_64 for a simulator that runs one of them: about twice the xcodebuild time, .app size and DerivedData (numbers in #1803).

Separately, the iOS simulator cache key carried no architecture, so an arm64-only Debug build, an Intel Mac's x86_64-only build and a fat build all shared <fp>-debug-sim, locally and in the Expo build cache provider.

Solution

One slice for --remote. The generic destination stays; the build adds ARCHS=<arch> ONLY_ACTIVE_ARCH=YES, the only thing that narrows a generic build. <arch> is the agent-device /health hostArch (callstack/agent-device#3048, upstream.hostArch first when talking to a proxy), read once per run. A missing or unknown value, an error, or no answer in 3 s means arm64, so today, before any agent-device release reports the field, every remote gets arm64.

EAS never gets a /health read: its daemon URL only exists once eas sim creates the session, which since #1212 happens after the build. EAS hosts are Apple silicon (checked in #1803), so arm64 is correct there.

Arch in the iOS simulator key, mirroring Android's ABI component:

Build	Key
local Debug simulator	<fp>-debug-sim-arm64 (-x86-64 on an Intel Mac, from process.arch)
--remote (any configuration)	<fp>-<config>-sim-<remote arch>
local Release/other simulator	<fp>-release-sim, unchanged
device	<fp>-debug-device, unchanged

Existing iOS Debug simulator cache entries miss once after this lands. A local non-Debug simulator build stays fat (#608), and I kept its key unsuffixed rather than adding -universal: no suffix already means "the ARCHS the project lists", as on Android, and Release entries stay valid. @stim-cli/expo-build-cache keys expo run:ios Debug simulator builds on the host arch the same way (--device generic stays unsuffixed, since it compiles every ARCHS; a simulator named with --device <name> keys as on-<name> with no arch, as before, because the name cannot tell a simulator from a phone), and remote build-cache providers now receive arch in runOptions, as Android passes abi.

stim ios --plan now refuses the ios.remote setting with STIM_BAD_ARG, as it already did for android.remote: the run keys the remote arch, which a plan cannot read without a session. No setting or flag is added.

Test plan

• Not run: a full stim ios --remote proxy install. A local proxy would let agent-device pick a simulator on this shared machine that Stim does not own, and an EAS session is billed.
• Real xcodebuild -showBuildSettings on apps/mobile (Xcode 27.0, arm64, target Stim, generic destination): Debug and Release both resolve ARCHS = arm64 x86_64; adding ARCHS=arm64 ONLY_ACTIVE_ARCH=YES gives arm64, and ARCHS=x86_64 ... gives x86_64.
• Real build with the exact new remote arguments (generic destination, ARCHS=arm64 ONLY_ACTIVE_ARCH=YES, fresh DerivedData, no compilation cache): BUILD SUCCEEDED, lipo -archs gives arm64 for Stim.app/Stim and ExpoCamera.framework, .app 172 MB, DerivedData 4.4 GB. The same build without the settings (what --remote did before): x86_64 arm64, 341 MB, 9.5 GB. Wall times were 339 s single-arch and 380 s fat, but the machine load differed a lot between the runs (load average about 80-225 vs 40-80), so the times don't compare; the A/B in ios --remote compiles arm64 and x86_64 for a one-arch simulator; iOS sim cache key has no arch #1803 measured 151 s vs 285 s.
• readRemoteSimulatorArch against a real loopback agent-device proxy 0.21.12: /agent-device/health returns no hostArch, and the reader returns arm64; a closed port also returns arm64.
• Unit tests cover the /health parser and reader (proxy upstream, absent, unknown arch, error, bad JSON, timeout), the xcodebuild argv with and without an arch, stim ios --remote proxy|eas reading /health once and building/keying that arch, the key table, and the CLI and expo-build-cache resolving the same iOS entries (cache-packages.test.ts).

Fixes #1803