blastradius: rename module to github.com/HexmosTech/git-lrc/blastradius

[LinceMathew]

Summary

Renames the blast-radius engine module from github.com/HexmosTech/blastradius to github.com/HexmosTech/git-lrc/blastradius, so other repos (LiveReview, for server-side blast radius on webhook/UI reviews) can import it with go get instead of a local replace.

• blastradius/go.mod module path
• root go.mod require + replace
• import paths in 33 Go files, plus code samples in docs/risk-scoring.md

No behavior change.

Verification

• go build ./... && go vet ./... in root and blastradius/
• go test: internal/appcore, tools 135 passed; blastradius 68 passed, 2 failing in score/ (TestFanInDecayAndDedup, TestFanInTransformCompressesHubDominance) fail identically on main (pre-existing)
• Scored the same 411-line diff with engine CLIs built from main and this branch: identical reports (only GeneratedAt and the order of tied ImpactedPackages differ)
• make build-local + fake-mode lrc review --blast-radius on a real range: 46/46 hunks scored
• go mod download github.com/HexmosTech/git-lrc/blastradius@510ed6d resolves from GitHub