Add a declarative user journey state machine and canonical diagnostic catalog for Playwright UI testing.

[copybara-service]

Add a declarative user journey state machine and canonical diagnostic catalog for Playwright UI testing.

Complex profiler frontend interactions require deterministic, reproducible end-to-end verification across varied hardware architectures (TPU v4/v5, TPU v6e) and distributed multi-host topologies. This change introduces a declarative state machine engine that models user diagnostic workflows as immutable sequences of state transitions.

Key Capabilities:

1. Declarative State Machine Engine:
   • Implements JourneyStep, JourneyScenario, and ActionType enum in test_user_journeys.py.
   • Dynamically dispatches actions (goto, switch_tool, select_host, go_back, go_forward) with automatic synchronization and error diagnostic context.
2. Canonical Diagnostic Journey Catalog:
   • Introduces tests/ui/journeys/diagnostic_journeys.json as the single source of truth containing 7 curated end-to-end diagnostic scenarios:
     • triage.pipeline_and_hardware_bounds: Performance bottleneck triage across Input Pipeline, HLO Op Stats, Op Profile, Roofline Model, and Trace Viewer.
     • memory.cross_hardware_tpu_to_v6e: Memory footprint and buffer allocation comparison between TPU v4/v5 and TPU v6e.
     • compiler.graph_and_multihost_scale: XLA compiler graph exploration and remote worker host trace inspection.
     • compute.op_profile_and_stats: Low-level kernel and op execution breakdown.
     • routing.popstate_resilience_loop: Browser back/forward popstate routing and state retention validation.
     • distributed.host_retention_and_trace: Host dropdown selection persistence across tool switches.
     • full_spectrum.multimodal_endurance: Full-breadth smoke sweep touching all primary tools.
3. Strict Invariant & Geometry Verification:
   • Enforces positive bounding box geometry on mounted components at every transition.
   • Verifies data table row population when requested by catalog steps.
   • Executes content poison-token sweeps at every waypoint to catch unrendered template tokens and raw error dumps.
4. Resilient Navigation Helpers:
   • Shared dropdown and Pythonic sidenav inspection helpers in tests/ui/ui_helpers.py.