feat: improve export status info

[LesnyRumcajs]

Adds pre-walk visibility logging for diff exports, a kind (snapshot/diff-snapshot/snapshot-gc) discriminator, and replaces the export status booleans with a state machine
(idle/running/succeeded/cancelled/failed) plus the failure error.

BREAKING CHANGE: Forest.ChainExportStatus no longer returns the exporting/cancelled/succeeded booleans; consumers must read the new state field (see scripts/tests/calibnet_export_check.sh for the migration).

Summary of changes

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