feat(diff): head each bulk turn with a run-log dispatch banner

[rickliujh]

Summary

A fanned-out run's diffs read as one flat wall: every entry repeated the whole breadcrumb inline ([≡ root ≡] › item › submodule) at the same weight as the diff body, so turn boundaries never stood out.

This heads each bulk turn the way the run log already announces a dispatch:

• an inverted ≡ root ≡ chip banner naming the turn, once per turn;
• a ▸ parent › child hand-off once per submodule path beneath it — root-free, so headers inside a turn read lighter;
• the target (repo) line once per repo.

DiffCollector.Print composes these with three-level dedup; DiffHeader composes the same pieces for full mode. A single-module run keeps its plain [name] chip + target header, byte-for-byte unchanged.

Before

[≡ bulk-deploy-k8s ≡] › deploy-k8s-1 › autocert
file://…/cluster-1 (main)
--- /dev/null
+++ issuer.yaml

After

[≡ bulk-deploy-k8s ≡] deploy-k8s-1
▸ deploy-k8s-1 › autocert
file://…/cluster-1 (main)
--- /dev/null
+++ issuer.yaml

(indigo chip + teal ▸ in a real terminal)

Testing

• go build ./..., go vet ./..., full go test ./... — all green.
• Updated the two bulk header tests; added TestDiffHeader_SubmoduleGetsRootFreeHandoff and TestDiffCollector_BannerOncePerTurnHandoffPerSubmodule.
• Drove the built binary against a nested bulk module (both loom diff and --quick) to confirm real output.

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