feat: add pack installation preflight

[9Ashwin]

Problem

Users need to inspect the cost and safety of selecting a sticker pack before any image download or library mutation.

Change

• add a source snapshot and installation preflight for an explicit pack ID
• verify the raw manifest revision, item count, declared bytes, local file size, both hashes, and format signature
• report the target, added/reused counts, and download bytes through packs install ID --dry-run
• reject source, revision, and digest conflicts without creating the home directory, lock, or catalog cache
• keep normal image download and install commit for the follow-up installation issue

Validation

• GOTOOLCHAIN=auto go test -race ./...
• GOTOOLCHAIN=auto go vet ./...
• GOTOOLCHAIN=auto CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build ./...
• GOTOOLCHAIN=auto go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.13.2 run ./...
• git diff --check and gofmt check