Fix dashboard diff line counts

[w287346141]

Summary

• fix dashboard git diff parsing so +++ / --- file header lines do not inflate addition/deletion counts
• detect added and deleted file blocks even when they are preceded by the diff --git header
• add a focused parser regression test covering modified, added, and deleted files

Validation

• npm.cmd test -- --run tests/git-diffs.test.ts
• npm.cmd test -- --run tests/git-diffs.test.ts tests/server-dashboard.test.ts
• npm.cmd run lint
• npm.cmd run typecheck
• git diff --check
• push hook ran npm.cmd run verify