Supernode enhancements, fixes

[j-rafique]

1. Per-RPC Upload Task Tracking — Shared state caused races; now uses per-RPC uploadHandle for concurrency safety. supernode/supernode/transport/grpc/cascade/register.go:32,105,117

2. Abort Work on Event Streaming Failures — Ignored send failures wasted work after client disconnect; now checks errors and stops immediately. supernode/supernode/cascade/task.go:21, register.go:62, download.go:51,106

3. RQStore First-Batch Flag Key Consistency — Wrong key prevented worker pickup; now uses taskID for proper symbol replication/deletion. supernode/supernode/adaptors/p2p.go:67,137, pkg/storage/rqstore/store.go:136, p2p/kademlia/rq_symbols.go:36

4. Unique Decode Workspace Per Download — Deterministic paths caused collisions; now uses <base>/downloads/<actionID>/<uuid> for isolation. supernode/pkg/codec/decode.go:55,60

5. Guaranteed Download Workspace Cleanup — Failures leaked workspaces; handler now defers cleanup to prevent disk leaks. supernode/supernode/transport/grpc/cascade/download.go:31,55, cascade/download.go:92

6. Zero-Copy Download Streaming — Per-chunk copies reduced throughput; now reuses buffer for higher performance. supernode/supernode/transport/grpc/cascade/download.go:90,94

7. Upload Temp Dir Cleanup on Errors — Early returns leaked temp dirs; now defers os.RemoveAll(tempDir) after creation. supernode/supernode/transport/grpc/cascade/register.go:38,44, handler_test.go:42

8. Reject Duplicate In-Flight Uploads — Concurrent calls doubled work; atomic tracker now fast-fails duplicates with AlreadyExists. supernode/pkg/task/task.go:33, handle.go:49, cascade/register.go:103,110

[roomote]

Rooviewer   See task on Roo Cloud

Review of changes since last review (5ca4a36...e8123cd):

The latest commit deletes supernode/adaptors/p2p_test.go, which was the test verifying that UpdateIsFirstBatchStored is called with taskID (not actionID). This removes test coverage for one of the key audit fixes in this PR. Consider keeping this test to prevent regressions.

• Deleted test file supernode/adaptors/p2p_test.go removes coverage for the taskID vs actionID fix

Previous reviews

• 5ca4a36: Review #1

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