feat(launchpack): add driver-independent packaging service

[joeykchen]

Summary

• add a reusable launchpack service that packages launchers without a local XGo driver
• resolve and verify runtime bundles, project payloads, source/native bridges, and generated launchers
• keep runtime acquisition and payload assembly behind a small configuration API

Verification

• go test -count=1 -race ./...
• go vet ./internal/launchpack
• Windows amd64 cross-compile of internal/launchpack
• gofmt and git diff checks

[fennoai]

Reviewed the complete fixed diff for runtime acquisition, project/payload assembly, source-bridge compilation, launcher generation, validation, cleanup, and cross-file identity contracts. I found no actionable PR-introduced correctness, security, performance, or compatibility issue meeting the review threshold.

Verification: go test -count=1 ./internal/launchpack, go test -race -count=1 ./internal/launchpack, go vet ./internal/launchpack, and git diff --check all pass. A full go test -count=1 ./... run reaches an existing failure in internal/projectpolicy (TestPortableConfigSnapshotRejectsIdentityAndContentDrift), outside this PR's changed files.