fix: derive geographic move directions

[christopherjnelson]

Summary

• derive move-option compass labels from great-circle bearings between H3 cell centers
• preserve the existing six-direction contract and legal target set
• cover sector boundaries, dateline wraparound, multiple locations and resolutions, and service integration
• document that H3 traversal order never determines direction

Scope

• observation-integrity direction fix only
• per-agent factual history remains the separate O1b follow-up

Validation

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