fix: deduplicate cookbook manifest files by path

[tas50]

Cookbook.AllFiles() concatenated the flat all_files manifest and all nine per-segment slices, on the strength of a comment asserting that a server populates "one shape or the other". Nothing guarantees that.

When both are populated, every file comes back twice — and Download turns each entry into a job handed to parallelForEach:

jobs[i] = fileDownload{url: ref.URL, dest: dest, path: ref.Path}

So two goroutines race to os.WriteFile the same destination, and each file is fetched from the bookshelf twice. Measured on a one-file cookbook listed in both shapes: 2 fetches, 2 unsynchronised writes.

Fixed by deduplicating on Path during the merge — all_files wins, and within the per-segment slices the first occurrence does.

Why the existing tests missed it

cinc-zero returns only all_files, so the integration suite can't reach this shape. The new test drives the merged manifest directly.

Test plan

• TestAllFiles_DedupesByPath — a manifest listing two files in both shapes returns 2 refs, not 4 (failed before: returned 4)
• TestDownload_FetchesEachFileOnce — counts bookshelf fetches (failed before: 2, want 1)
• go vet ./... clean
• go test ./... -race -count=2 — the race detector is the point here
• cd integration && go test ./...
• No public surface change, so no README Status update needed