Add route outcome diagnostics

[guelfoweb]

Summary:

• Adds env-gated non-content route outcome diagnostics under ORBIT_KV_DIAG.
• Emits kv_diag_route_outcome events correlated with request_id, model_call_id, pass_index, phase, and tools_mode.
• Classifies route outcomes as route_direct_final_stop, route_no_decision_length_retry, route_parsed_tool, route_parsed_chat, route_invalid_output, or route_other_retry.
• Logs only metadata: finish reason, decision type, output length/token count, and retry reason.
• Does not log raw prompt, raw route output, raw tool output, or user-visible content.
• Documents the metric in docs/ROUTE_OUTCOME_OBSERVABILITY.md.

Validation:

• PYTHONPATH=src python3 -m unittest tests.test_kv_diag -q: PASS
• python3 -m unittest discover -s tests -q: PASS
• python3 -m compileall -q src tests scripts: PASS
• git diff --check: PASS

Scope:

• Diagnostics only
• No prompt changes
• No routing policy changes
• No tool-selection changes
• No backend/native/MTP changes
• No final policy changes
• No KV/cache behavior changes
• No visible behavior changes