feat: add GA route core runtime

[abraxas914]

Context: PR109 mixed route core, GUI, launcher, and API surface into one review unit. This PR restarts with the backend-only slice: structured provider/route storage, failover routing, active-route switching, explicit reload, diagnostics, and legacy import. GUI, launcher, and API server stay out of scope and will be handled separately if this backend contract is accepted.

Included

• ga_switch backend core: models, store, service, diagnostics, testing
• minimal agentmain.py hooks: set_active_route(), reload_llm_config(), describe_llms()
• minimal llmcore.py metadata needed for failover and diagnostics
• canonical backend snapshot via get_runtime_snapshot()
• focused backend test suite in tests/test_ga_switch.py

Excluded

• any frontends/*
• launch.pyw
• api_server.py and API server docs/scripts
• UI/viewmodel payload builders and handoff/audit docs

Hotfix semantics

• route/provider updates only affect future tasks
• set_active_route() does not interrupt an in-flight task
• reload_llm_config() still fails loudly while the agent is running
• no watcher-based auto reload or shared runtime changes

Validation

• D:\Workspace\pixi-analysis-r-py\.pixi\envs\default\python.exe -m unittest tests.test_ga_switch