feat(deploy): stage streamed payloads to a temp file for replication

[kriszyp]

Stacked on #531 — please review #530 and #531 first; this PR will be retargeted to `main` after both land.

Summary

Origin-side payload staging needed for the direct-HTTPS peer relay (harper-pro slice 3b). When a `deploy_component` request will be replicated to peers, the origin now writes the streamed payload to a temp file before local extraction so it can re-stream the same bytes to each peer afterwards. Without this, the payload Readable is consumed once by local extract and gone by the time replication runs.

Third slice of #524.

Where to look

• `components/payloadStaging.ts` — single-purpose utility that pipelines a Readable into a temp `payload.tar.gz` and returns a cleanup callback. The trade-off comment in the file header explains why we write-then-read instead of teeing in flight (simpler code, identical backpressure for extraction; disk speed isn't the bottleneck).
• `components/operations.js` — staging gate in `deployComponent`. Skipped when:
  • the deploy is package-identifier-based (`req.package`, no `payload` stream),
  • `req.replicated === false` (the operation is itself a replicated deploy that the peer received, so no further fan-out),
  • or there are no peers (`server.nodes.length === 0` — single-node deploys keep their zero-disk-copy property).
• Cleanup runs in a `finally` after `server.replication.replicateOperation(req)`, plus a defensive cleanup if `prepareApplication` fails before replication.

Backward compatibility

• The staging is invisible to the operations API contract: `req.payload` is replaced internally with a `createReadStream` over the temp file before the `Application` is constructed, so every downstream code path (`Application.extractApplication`, the SSE phase emitter, anything reading `application.payload`) keeps seeing a `Readable` exactly as before.
• Single-node and package-identifier deploys: zero change in behavior, no temp file written.

Test plan

• `npx mocha unitTests/components/payloadStaging.test.js` — 5/5 pass (round-trip contents, cleanup removes file+dir, double-cleanup is safe, path-traversal sanitization keeps the staged dir under tmpdir, source-stream errors propagate).
• `npx mocha unitTests/server/fastifyRoutes/operations.test.js` — 21/21 pass (existing deployComponent tests; the staging branch is only entered when nodes are present, which these tests don't simulate).
• `npx mocha unitTests/components/packageComponent.test.js unitTests/server/serverHelpers/multipartParser.test.js unitTests/bin/multipartBuilder.test.js unitTests/server/serverHelpers/progressEmitter.test.js unitTests/bin/sseConsumer.test.js` — 32/32 pass (slice 1 + slice 2 unaffected).
• End-to-end replicated deploy — exercised in harper-pro's slice 3b integration test, not here.

🤖 Generated with Claude Code

[claude]

Reviewed; no blockers found.