perf(store): avoid loading headers while finding boundaries

[loglapa]

Closes #332

Overview

• scan contiguous head and tail ranges with height-index presence checks
• load and unmarshal only the final boundary header after the scan
• include pending headers and cached height indexes in presence checks
• keep the current boundary unchanged on datastore errors or cancellation
• guard height arithmetic at both uint64 limits
• assert datastore read counts for head and tail scans

Validation

• go test ./store -count=1
• go test -race ./store -count=1
• golangci-lint run

go test ./... reaches the existing p2p/TestExchangeServer_partialRangeNotExpanded panic on current main. That unrelated regression predates this branch and is fixed by #409; all other packages, including the changed store package, pass.