Add Appium parallel execution support

[omnarayan]

Summary

• Enable --parallel N for the Appium driver — all N sessions hit the same --appium-url with identical capabilities, server (local or cloud) allocates devices
• Per-worker cloud provider detection and result reporting (each session = separate cloud job)
• Reuses existing ParallelRunner work-queue infrastructure — no changes to the executor layer

Usage

# 3 parallel sessions against same Appium URL
maestro-runner --driver appium --appium-url "http://localhost:4723" \
  --caps caps.json test --parallel 3 flows/

# Cloud — same thing, provider allocates devices
maestro-runner --driver appium --appium-url "https://ondemand.saucelabs.com/wd/hub" \
  --caps caps.json test --parallel 3 flows/

Test plan

• go build ./... passes
• go test ./pkg/cli/... ./pkg/executor/... passes
• E2E: Android real device + emulator with --parallel 2 — flows distributed across both devices
• E2E: Single session (--parallel 1 / no flag) — unchanged behavior
• E2E: Cloud provider (Sauce Labs) with --parallel 2

Notes

• iOS local Appium parallel requires appium:udid per session — Appium XCUITest driver doesn't auto-distribute across booted simulators with identical caps
• Android local Appium parallel works automatically — UiAutomator2 distributes across available devices