fix(rpiv-pi): close build's validate-loop livelock — classifying gate, structured blockers, remediation progress digest

Run 2026-08-22_12-14-12-64eb: validate failed on whole-plan gates recorded
only in report prose (all risk rulings passed), remediate drift-escaped
without an edit, and the unchanged tree re-validated identically until the
backward-jump guard halted the run at reconcile. Three seams close it:
validateGate routes a fail to validate-fix only when a remediable handle
exists (pass:false ruling or blockers entry) and STOPs with a note
otherwise; validate emits attributed whole-plan failures as
blockers:[{id,command,file,line}] frontmatter that remediate's fixable
partition now accepts (the command is the procedure); remediationOutcome
snapshots a git-only tree digest around validate-fix and its edge STOPs on
an explicitly unchanged tree.