Add chronological sorting for ADIF exports

[rsuzuki0]

Summary

• sort SOTA-ready ADIF exports chronologically by date and time automatically
• add an opt-in FLEcli adif --sort flag for POTA, WWFF, and general ADIF exports
• preserve source order for records with identical timestamps and never modify the FLE input file
• warn when an unsorted non-SOTA export contains backward date/time transitions
• document the behavior and add unit plus end-to-end coverage

Motivation

The SOTA importer requires ADIF records to be chronological, so the program-specific --sota option now guarantees importer-compatible ordering without an additional flag. FLE shorthand can legitimately produce out-of-order timestamps when a handwritten multi-band activation is transcribed in page order. Other export types retain their existing order unless --sort is requested.

Validation

• go test ./...
• go test -race -count=1 ./...
• go vet ./...
• manual end-to-end test on a real-world multi-band activation file