fix(cascade): persist metadata-only first batch when no symbols selected

[j-rafique]

Summary

This PR fixes a first-batch persistence gap in cascade artefact storage.
Previously, storeCascadeSymbolsAndData only sent metadata (IDFiles) inside the symbol batching loop.
If no symbols were selected (len(keys) == 0), the loop never ran, but UpdateIsFirstBatchStored was still called.
That could mark the action as first-batch stored without actually persisting index/layout metadata to P2P.

What changed

• Added an explicit metadata-only batch path in storeCascadeSymbolsAndData:
• Triggered when len(keys) == 0 && len(metadataFiles) > 0
• Sends metadata via p2p.StoreBatch(...)
• Preserves timeout handling and structured logging
• Updates byte accounting consistently
• Existing first-batch flag update behavior now correctly happens only after successful metadata persistence.

Tests added

In supernode/adaptors/p2p_test.go:

TestStoreCascadeSymbolsAndData_MetadataOnlyBatchWhenNoSymbols

• Verifies metadata is sent when no symbols are selected
• Verifies call count and payload expectations
• Verifies success path (no symbols stored, no total symbols)

TestStoreCascadeSymbolsAndData_MetadataOnlyBatchFailureSkipsFirstBatchFlag

• Verifies metadata-only store errors are surfaced
• Verifies first-batch flag is not updated when metadata store fails

Why this matters

Ensures first-batch completion semantics remain correct and durable by guaranteeing metadata is persisted even when symbol selection yields zero symbols.

[roomote-v0]

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