ALL-1866: Investigate GEOCODE_FAILURE false positives (read-only)

Read-only investigation. Pull sample GEOCODE_FAILURE issues from production via psql/Datadog. Classify patterns (relevance threshold too strict? street-number regex rejecting valid formats? Mapbox query construction? transient errors recorded as permanent?). Files to read: domains/identity/src/services/staging/geocodeService.ts, profiles/commonChecks.ts, domains/location/src/services/geocoding/index.ts, utilities.ts. Output: a written investigation report identifying the dominant root cause(s), proposed fix scope, and risk assessment. NO code changes in this pass. Worktree: /home/agent/agents/wilbo/mono-all-1866-geocode-investigation.