feat: add deterministic segment target planner

[duck8823]

Summary

• plan the smallest contiguous Hot prefix against a captured Catalog snapshot and policy caps
• bind reservations to ordered canonical source evidence and freeze reserved source history
• persist measured cap-failure proofs for append-only release and strictly smaller retries
• normalize all deadline paths to typed selection-incomplete rollback
• add migration 46 and concurrency, Unicode byte-boundary, freeze, retry-proof, and timeout regressions

Validation

• go test ./... -count=1
• go vet ./...
• go tool golangci-lint run --timeout=5m
• focused race tests
• independent GPT-5.6 Sol review: APPROVE

Closes #1662