feat: run multiple devices per workspace with named slots

[janicduplessis]

Description

Each platform currently has one device assignment per workspace. Testing on an iPhone, an iPad, and hardware needs independent targets that subsequent commands can reuse.

Solution

Add --slot <name> to local iOS/Android runs, including physical devices and multiple simulators of the same model. Existing assignments remain in the default slot. Slots share Metro and build caches; native CLI runs serialize workspace mutations to protect shared build output.

Logs and status identify slots. stop --slot preserves Metro and sibling devices; workspace cleanup handles every slot.

Limits to review:

• Shared Metro requests cannot verify an individual simulator launch when slots coexist; Debug may report unverified.
• Reload remains platform-wide, and named remote sessions are unsupported.
• Upgrades retain ownership state. Use a slot-aware CLI consistently while named assignments exist.

Depends on #769, above #767 and #764.

Test plan

Regression coverage includes allocation, reuse, scoped cleanup, leases, log markers, launch attribution, default-state compatibility, and EAS-downloaded builds in default and named slots.

The combined slot stack and QA fixes passed the full loop and cache suites for bare React Native and Expo on both platforms. iOS ran locally on the 16 GB Mac; all four Android jobs passed in the hosted matrix. Both iOS pool suites also passed hosted QA.

Both iOS loops exercised three distinct simulators (two iPhones and an iPad), reuse, cache hits, attributed logs, scoped stop/resume, and complete cleanup. Each iOS cache suite passed all seven applicable checks, including one build shared by two racing workspaces. The initial failures led to readiness fixes in #779, Android cleanup fixes in #780, and selecting an iPad supported by an available runtime in this PR.

Physical-device installs were not exercised on hardware. Lease and command behavior have regression coverage. The earlier full hosted run contains iOS failures; the passing sign-off combines its Android jobs with the final local iOS loops/caches and hosted pool run.

Fixes #770.