fix(diff): separate every bulk header block with a blank line

[rickliujh]

Summary

Follow-up to #33. The module breadcrumb between diffs wasn't visible enough: inside a turn, a submodule's ▸ parent › child hand-off (and a bare target change) printed glued to the end of the diff above it — only the turn banner was set off by a blank line. So a breadcrumb sitting between two diffs got lost in the surrounding diff text.

This leads every header block with a blank line, not just the turn banner — the separator moves out of diffTurnBanner and into the callers (DiffCollector.Print and DiffHeader), emitted whenever a new header block starts.

Before (within a turn)

+kind: ClusterIssuer
+duration: 720h
▸ deploy-k8s-1 › ingress      ← glued to the diff above, easy to miss
file://…/cluster-1 (main)

After

+kind: ClusterIssuer
+duration: 720h

▸ deploy-k8s-1 › ingress      ← blank line sets it apart
file://…/cluster-1 (main)

The rendered header bytes are otherwise unchanged, so full mode and the single-module path are unaffected.

Testing

• go build ./..., go vet ./..., full go test ./... — all green.
• Added a within-turn separation assertion to TestDiffCollector_BannerOncePerTurnHandoffPerSubmodule.
• Drove the built binary against a nested bulk module with two submodules per turn to confirm the real output.

🤖 Generated with Claude Code